[BUG] Incorrect padding error with Denon AVR

[JSubelj]

Describe the bug
When I try to browse like opening artists, albums etc. on my Denon AVR X1600H, I get an error - server communication fail (dosen't really have any additional info) but in docker logs I get the following:

...
upmpdcli-navidrome  | CMDTALK: subsonic-app.py: '2026-01-18 15:40:54.342376 browse executed (show_tag_entries) collecting [5] entries in [0.185]'
upmpdcli-navidrome  | CMDTALK: subsonic-app.py: "2026-01-18 15:40:58.869237 browse: args: --{'cmdtalk:proc': 'browse', 'count': '100', 'offset': '0', 'flag': 'children', 'objid': '0$subsonic$/eyJuIjogImEiLCAidiI6ICJvIn0'}--"
upmpdcli-navidrome  | CMDTALK: subsonic-app.py: "2026-01-18 15:40:58.869470 Could not decode [eyJuIjogImEiLCAidiI6ICJvIn0] [<class 'binascii.Error'>] [Incorrect padding]"
upmpdcli-navidrome  | CMDTALK: subsonic-app.py: 'processmessage: processor raised: [Incorrect padding]'
upmpdcli-navidrome  | Traceback (most recent call last):
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/pycommon/cmdtalk.py", line 170, in processmessage
upmpdcli-navidrome  |     outfields = processor.process(params)
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/pycommon/cmdtalkplugin.py", line 67, in process
upmpdcli-navidrome  |     return self.dispatcher.run(params[prcnmkey], params)
upmpdcli-navidrome  |            ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/pycommon/cmdtalkplugin.py", line 51, in run
upmpdcli-navidrome  |     return func(params)
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/subsonic/subsonic-app.py", line 3076, in browse
upmpdcli-navidrome  |     decoded_path: str = codec.decode(last_path_item)
upmpdcli-navidrome  |                         ~~~~~~~~~~~~^^^^^^^^^^^^^^^^
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/subsonic/codec.py", line 28, in decode
upmpdcli-navidrome  |     message_bytes: bytes = base64.b64decode(base64_bytes)
upmpdcli-navidrome  |                            ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
upmpdcli-navidrome  |   File "/usr/lib/python3.13/base64.py", line 88, in b64decode
upmpdcli-navidrome  |     return binascii.a2b_base64(s, strict_mode=validate)
upmpdcli-navidrome  |            ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
upmpdcli-navidrome  | binascii.Error: Incorrect padding
upmpdcli-navidrome  | :2:../src/mediaserver/cdplugins/plgwithslave.cxx:596::PlgWithSlave::browse: slave failure

As I see it, the problem is that Denon sends incorrectly padded base64 string and python fails to translate it. Is there a workaround? I can't change Denon's behaviour (even tho I think that Denon is definitely to blame).

To Reproduce
Steps to reproduce the behavior:

1. Run server
2. On Denon AVR go to any category
3. See error

Expected behavior
Server serving data (even tho base64 is incorrectly padded).

Docker run command
docker compose up

Docker-compose.yaml file

 upmpdcli:
        image: giof71/upmpdcli:latest
        container_name: upmpdcli-navidrome
        network_mode: host
        depends_on:
            - navidrome
        environment:
            - RENDERER_MODE=NONE
            - FRIENDLY_NAME=navidrome-dlna
            - SUBSONIC_ENABLE=yes
            - SUBSONIC_AUTOSTART=yes
            - SUBSONIC_BASE_URL=${SUBSONIC_BASE_URL}
            - SUBSONIC_PORT=${SUBSONIC_PORT}
            - SUBSONIC_USER=${SUBSONIC_USER}
            - SUBSONIC_PASSWORD=${SUBSONIC_PASSWORD}
            - SUBSONIC_APPEND_YEAR_TO_ALBUM=Yes #optional
            - SUBSONIC_APPEND_CODECS_TO_ALBUM=Yes #optional
            - SUBSONIC_DOWNLOAD_PLUGIN=yes #optional, will download updated plugin from the specified branch
            - SUBSONIC_PLUGIN_BRANCH=latest #optional, it is the branch to be downloaded if SUBSONIC_DOWNLOAD_PLUGIN is set to yes
        restart: unless-stopped

Screenshots
If applicable, add screenshots to help explain your problem.

Operating system(please complete the following information):

• OS: Debian + Docker
• Architecture: amd64
• Version: latest (stable-full-2026-01-09)

Additional context
Thank you for your help.

[GioF71]

Looks like the control point is stripping the trailing "==" characters. It's probably something we can handle somehow.
Can you test with BubbleUPnP, just to isolate the issue?

[GioF71]

You might want to try the following as a patch.
Create a file named codec.py, maybe where your docker-compose.yaml file is.
The contents of the file:

import base64
import constants
import config
from msgproc_provider import msgproc


__encoding: str = "utf-8"


def encode(name: str) -> str:
    message_bytes: bytes = name.encode(__encoding)
    base64_bytes: bytes = base64.b64encode(message_bytes)
    id: str = base64_bytes.decode(__encoding)
    # reproduce issue: https://github.com/GioF71/upmpdcli-docker/issues/608
    # while id.endswith("="):
    #     id = id[0:len(id) - 1]
    return id


def decode(id: str) -> str:
    missing_padding: int = len(id) % 4
    data: str = id
    if missing_padding != 0:
        # add missing padding characters
        data += '=' * (4 - missing_padding)
        if config.get_config_param_as_bool(constants.ConfigParam.VERBOSE_LOGGING):
            msgproc.log(f"codec.decode sanitized [{data}] "
                        f"(added [{4 - missing_padding}] padding characters)")
    base64_bytes: bytes = data.encode(__encoding)
    message_bytes: bytes = base64.b64decode(base64_bytes)
    name: str = message_bytes.decode(__encoding)
    return name

Mount the file as /usr/share/upmpdcli/cdplugins/subsonic/codec.py, adding this line under "- volumes:"

volumes:
  - ./codec.py:/usr/share/upmpdcli/cdplugins/subsonic/codec.py:ro

so your service upmpdcli might look like this in your compose file:

upmpdcli:
    image: giof71/upmpdcli:latest
    container_name: upmpdcli-navidrome
    network_mode: host
    depends_on:
        - navidrome
    environment:
        - RENDERER_MODE=NONE
        - FRIENDLY_NAME=navidrome-dlna
        - SUBSONIC_ENABLE=yes
        - SUBSONIC_AUTOSTART=yes
        - SUBSONIC_BASE_URL=${SUBSONIC_BASE_URL}
        - SUBSONIC_PORT=${SUBSONIC_PORT}
        - SUBSONIC_USER=${SUBSONIC_USER}
        - SUBSONIC_PASSWORD=${SUBSONIC_PASSWORD}
        - SUBSONIC_APPEND_YEAR_TO_ALBUM=Yes #optional
        - SUBSONIC_APPEND_CODECS_TO_ALBUM=Yes #optional
    volumes:
        - ./codec.py:/usr/share/upmpdcli/cdplugins/subsonic/codec.py:ro
    restart: unless-stopped

I have removed SUBSONIC_DOWNLOAD_PLUGIN and SUBSONIC_PLUGIN_BRANCH, now deprecated.

Let me know if this helps!

[JSubelj]

Thanks! It works with custom codec.py. I now get this errors, while looking up albums. Don't really know if they are related and I think I also get them on bubble, but because I'm comparing side by side I can't really tell:

"2026-01-19 20:39:54.798653 browse: args: --{'cmdtalk:proc': 'browse', 'count': '100', 'offset': '0', 'flag': 'children', 'objid': '0$subsonic$/eyJuIjogImEiLCAidiI6ICJvIn0/eyJuIjogImEiLCAidiI6ICJ0In0/eyJuIjogImYiLCAidiI6ICIwNmNWdWd0dHNCUGZvUFhmaGRnTDFGIn0/eyJuIjogInQiLCAidiI6ICIwNmNWdWd0dHNCUGZvUFhmaGRnTDFGIn0/eyJuIjogIm8iLCAidiI6ICIycVRhSzN0NjVDeDlvUzZuVkoxZmpaIiwgInNrcHJ0c3RkIjogIjA'}--"
upmpdcli-navidrome  | CMDTALK: subsonic-app.py: 'processmessage: processor raised: [Unterminated string starting at: line 1 column 55 (char 54)]'
upmpdcli-navidrome  | Traceback (most recent call last):
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/pycommon/cmdtalk.py", line 170, in processmessage
upmpdcli-navidrome  |     outfields = processor.process(params)
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/pycommon/cmdtalkplugin.py", line 67, in process
upmpdcli-navidrome  |     return self.dispatcher.run(params[prcnmkey], params)
upmpdcli-navidrome  |            ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/pycommon/cmdtalkplugin.py", line 51, in run
upmpdcli-navidrome  |     return func(params)
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/subsonic/subsonic-app.py", line 3077, in browse
upmpdcli-navidrome  |     item_dict: dict[str, any] = json.loads(decoded_path)
upmpdcli-navidrome  |                                 ~~~~~~~~~~^^^^^^^^^^^^^^
upmpdcli-navidrome  |   File "/usr/lib/python3.13/json/__init__.py", line 346, in loads
upmpdcli-navidrome  |     return _default_decoder.decode(s)
upmpdcli-navidrome  |            ~~~~~~~~~~~~~~~~~~~~~~~^^^
upmpdcli-navidrome  |   File "/usr/lib/python3.13/json/decoder.py", line 345, in decode
upmpdcli-navidrome  |     obj, end = self.raw_decode(s, idx=_w(s, 0).end())
upmpdcli-navidrome  |                ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
upmpdcli-navidrome  |   File "/usr/lib/python3.13/json/decoder.py", line 361, in raw_decode
upmpdcli-navidrome  |     obj, end = self.scan_once(s, idx)
upmpdcli-navidrome  |                ~~~~~~~~~~~~~~^^^^^^^^
upmpdcli-navidrome  | json.decoder.JSONDecodeError: Unterminated string starting at: line 1 column 55 (char 54)
upmpdcli-navidrome  | :2:../src/mediaserver/cdplugins/plgwithslave.cxx:596::PlgWithSlave::browse: slave failure
upmpdcli-navidrome  | CMDTALK: subsonic-app.py: "2026-01-19 20:39:54.808230 browse: args: --{'cmdtalk:proc': 'browse', 'count': '100', 'offset': '0', 'flag': 'children', 'objid': '0$subsonic$/eyJuIjogImEiLCAidiI6ICJvIn0/eyJuIjogImEiLCAidiI6ICJ0In0/eyJuIjogImYiLCAidiI6ICIwNmNWdWd0dHNCUGZvUFhmaGRnTDFGIn0/eyJuIjogInQiLCAidiI6ICIwNmNWdWd0dHNCUGZvUFhmaGRnTDFGIn0/eyJuIjogIm8iLCAidiI6ICIycVRhSzN0NjVDeDlvUzZuVkoxZmpaIiwgInNrcHJ0c3RkIjogIjA'}--"
upmpdcli-navidrome  | CMDTALK: subsonic-app.py: 'processmessage: processor raised: [Unterminated string starting at: line 1 column 55 (char 54)]'
upmpdcli-navidrome  | Traceback (most recent call last):
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/pycommon/cmdtalk.py", line 170, in processmessage
upmpdcli-navidrome  |     outfields = processor.process(params)
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/pycommon/cmdtalkplugin.py", line 67, in process
upmpdcli-navidrome  |     return self.dispatcher.run(params[prcnmkey], params)
upmpdcli-navidrome  |            ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/pycommon/cmdtalkplugin.py", line 51, in run
upmpdcli-navidrome  |     return func(params)
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/subsonic/subsonic-app.py", line 3077, in browse
upmpdcli-navidrome  |     item_dict: dict[str, any] = json.loads(decoded_path)
upmpdcli-navidrome  |                                 ~~~~~~~~~~^^^^^^^^^^^^^^
upmpdcli-navidrome  |   File "/usr/lib/python3.13/json/__init__.py", line 346, in loads
upmpdcli-navidrome  |     return _default_decoder.decode(s)
upmpdcli-navidrome  |            ~~~~~~~~~~~~~~~~~~~~~~~^^^
upmpdcli-navidrome  |   File "/usr/lib/python3.13/json/decoder.py", line 345, in decode
upmpdcli-navidrome  |     obj, end = self.raw_decode(s, idx=_w(s, 0).end())
upmpdcli-navidrome  |                ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
upmpdcli-navidrome  |   File "/usr/lib/python3.13/json/decoder.py", line 361, in raw_decode
upmpdcli-navidrome  |     obj, end = self.scan_once(s, idx)
upmpdcli-navidrome  |                ~~~~~~~~~~~~~~^^^^^^^^
upmpdcli-navidrome  | json.decoder.JSONDecodeError: Unterminated string starting at: line 1 column 55 (char 54)
upmpdcli-navidrome  | :2:../src/mediaserver/cdplugins/plgwithslave.cxx:596::PlgWithSlave::browse: slave failure
...
upmpdcli-navidrome  | CMDTALK: subsonic-app.py: "2026-01-19 20:44:49.522246 browse: args: --{'cmdtalk:proc': 'browse', 'count': '100', 'offset': '0', 'flag': 'children', 'objid': '0$subsonic$/eyJuIjogImEiLCAidiI6ICJhIn0/eyJuIjogImEiLCAidiI6ICJmIn0/eyJuIjogIm8iLCAidiI6ICI3aDU3cnVHR1oxUjlIVDlDQU40WVY4In0/eyJuIjogImYiLCAidiI6ICIzbmtVOWRSYmhnbFJudU5nSmJYWG5SIiwgImxibWlkcmY0cnRzdCI6ICI3aDU3cnVHR1oxUjlIVDlDQU40WVY4In0/eyJuIjogInQiLCAidiI'}--"
upmpdcli-navidrome  | CMDTALK: subsonic-app.py: "processmessage: processor raised: [Expecting ':' delimiter: line 1 column 15 (char 14)]"
upmpdcli-navidrome  | Traceback (most recent call last):
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/pycommon/cmdtalk.py", line 170, in processmessage
upmpdcli-navidrome  |     outfields = processor.process(params)
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/pycommon/cmdtalkplugin.py", line 67, in process
upmpdcli-navidrome  |     return self.dispatcher.run(params[prcnmkey], params)
upmpdcli-navidrome  |            ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/pycommon/cmdtalkplugin.py", line 51, in run
upmpdcli-navidrome  |     return func(params)
upmpdcli-navidrome  |   File "/usr/share/upmpdcli/cdplugins/subsonic/subsonic-app.py", line 3077, in browse
upmpdcli-navidrome  |     item_dict: dict[str, any] = json.loads(decoded_path)
upmpdcli-navidrome  |                                 ~~~~~~~~~~^^^^^^^^^^^^^^
upmpdcli-navidrome  |   File "/usr/lib/python3.13/json/__init__.py", line 346, in loads
upmpdcli-navidrome  |     return _default_decoder.decode(s)
upmpdcli-navidrome  |            ~~~~~~~~~~~~~~~~~~~~~~~^^^
upmpdcli-navidrome  |   File "/usr/lib/python3.13/json/decoder.py", line 345, in decode
upmpdcli-navidrome  |     obj, end = self.raw_decode(s, idx=_w(s, 0).end())
upmpdcli-navidrome  |                ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
upmpdcli-navidrome  |   File "/usr/lib/python3.13/json/decoder.py", line 361, in raw_decode
upmpdcli-navidrome  |     obj, end = self.scan_once(s, idx)
upmpdcli-navidrome  |                ~~~~~~~~~~~~~~^^^^^^^^
upmpdcli-navidrome  | json.decoder.JSONDecodeError: Expecting ':' delimiter: line 1 column 15 (char 14)
upmpdcli-navidrome  | :2:../src/mediaserver/cdplugins/plgwithslave.cxx:596::PlgWithSlave::browse: slave failure

Don't know if this errors are related to my initial issue tho. I think the issue is resolved by modified codec.py :). For those errors maybe you have some ideas, otherwise I'm gonna try and figure out at least how to repeat them and then maybe open a new issue, when I'll have more time.

Thank you for your help and amazing responsiveness!

[GioF71]

Are you sure that this also happens with BubbleUPnP?
The control point seems to be truncating data. The strings are:

eyJuIjogImEiLCAidiI6ICJvIn0 -> decodes to {"n": "a", "v": "o"}
eyJuIjogImEiLCAidiI6ICJ0In0 -> decodes to {"n": "a", "v": "t"}
eyJuIjogImYiLCAidiI6ICIwNmNWdWd0dHNCUGZvUFhmaGRnTDFGIn0 -> decodes to {"n": "f", "v": "06cVugttsBPfoPXfhdgL1F"}
eyJuIjogInQiLCAidiI6ICIwNmNWdWd0dHNCUGZvUFhmaGRnTDFGIn0 -> decodes to {"n": "t", "v": "06cVugttsBPfoPXfhdgL1F"}
eyJuIjogIm8iLCAidiI6ICIycVRhSzN0NjVDeDlvUzZuVkoxZmpaIiwgInNrcHJ0c3RkIjogIjA -> decodes to {"n": "o", "v": "2qTaK3t65Cx9oS6nVJ1fjZ", "skprtstd": "0

With the last string, the base64 decoding does not return a valid json message.

The string had to probably be: eyJuIjogIm8iLCAidiI6ICIycVRhSzN0NjVDeDlvUzZuVkoxZmpaIiwgInNrcHJ0c3RkIjogIjAifQ==
which would decode to {"n": "o", "v": "2qTaK3t65Cx9oS6nVJ1fjZ", "skprtstd": "0"}

I might try to reduce the last key string (skprtstd) length, but eventually, if album ids get longer, we will hit some limit with the control point.
Can you please test BubbleUPnP, and see if at least browsing works properly?

Weird enough, the whole truncated string is exactly 256 characters:

0$subsonic$/eyJuIjogImEiLCAidiI6ICJvIn0/eyJuIjogImEiLCAidiI6ICJ0In0/eyJuIjogImYiLCAidiI6ICIwNmNWdWd0dHNCUGZvUFhmaGRnTDFGIn0/eyJuIjogInQiLCAidiI6ICIwNmNWdWd0dHNCUGZvUFhmaGRnTDFGIn0/eyJuIjogIm8iLCAidiI6ICIycVRhSzN0NjVDeDlvUzZuVkoxZmpaIiwgInNrcHJ0c3RkIjogIjA

Looks like the limit is quite low. I wonder probably Minimserver, if the path are nested enough, might fail as well with this control point as well?

[JSubelj]

You are right. Just checked again and this does not happen with BubbleUpnp (or at least after browsing around I could not reproduce).

[GioF71]

Good to hear that. Does it also play?

[JSubelj]

Yes. BubbleUpnp everything looks like it works.

[GioF71]

Hello, can you try this updated codec.py?
It might solve the Denon AVR issue by minimizing the length of those strings.
The price is that there is a growing cache so it consumes a little memory.
But we can find a more elegant solution if this patch works.

import base64
import constants
import config
from msgproc_provider import msgproc


__encoding: str = "utf-8"

__by_id: dict[str, str] = {}
__by_encoded: dict[str, str] = {}


def encode(data: str) -> str:
    encoded_name: str = __encode(data)
    # msgproc.log(f"codec.encode for name [{data}] -> [{encoded_name}]")
    if encoded_name in __by_encoded:
        # msgproc.log(f"codec.encode [{encoded_name}] -> [{__by_encoded[encoded_name]}]")
        return __by_encoded[encoded_name]
    new_id: str = str(len(__by_id) + 1)
    # msgproc.log(f"codec.encode new_id [{new_id}] for data [{data}] -> encoded to: [{encoded_name}]")
    __by_id[new_id] = encoded_name
    __by_encoded[encoded_name] = new_id
    return str(new_id)


def decode(id: str) -> str:
    msgproc.log(f"codec.decode for id [{id}]")
    # must be in dict, otherwise we get an exception
    encoded: str = __by_id[id] if id in __by_id else None
    if encoded is None:
        raise Exception(f"codec.decode id [{id}] not found")
    # decode encoded
    return __decode(encoded)


def __encode(name: str) -> str:
    message_bytes: bytes = name.encode(__encoding)
    base64_bytes: bytes = base64.b64encode(message_bytes)
    id: str = base64_bytes.decode(__encoding)
    return id


def __decode(id: str) -> str:
    missing_padding: int = len(id) % 4
    data: str = id
    if missing_padding != 0:
        # add missing padding characters
        data += '=' * (4 - missing_padding)
        if config.get_config_param_as_bool(constants.ConfigParam.VERBOSE_LOGGING):
            msgproc.log(f"codec.decode sanitized [{data}] "
                        f"(added [{4 - missing_padding}] padding characters)")
    base64_bytes: bytes = data.encode(__encoding)
    message_bytes: bytes = base64.b64decode(base64_bytes)
    name: str = message_bytes.decode(__encoding)
    return name

[JSubelj]

After some use I have not seen any errors, so I think this works :D Will the cache also speedup searching after some use?

[GioF71]

After some use I have not seen any errors, so I think this works :D Will the cache also speedup searching after some use?

Very nice to hear this works for you!

Hello, actually this kind of cache does not affect speed. I have just avoided to have those long strings, by providing integer values instead and keeping track of the corresponding dictionaries (the things I obtain when decoding those long strings). So this only improves compatibility with control points. Some don't play well with those longs strings. Yours is not the only case, another user with a Yamaha device had a similar issue.

This modality will be default in the next release. I will notify you here, it will be a new image with the "edge" tag with subsonic plugin 0.8.30 (it's not out yet). When you will get this (upcoming) image, or a "latest" image which will also includes version 0.8.30 or above, you will have to remove the mounted codec.py file.