Not working from external network

[egeu5]

Hello,
I have a server which is available over IPv6 only from the web. There is also a V-Server in the datacenter with a public v4 and v6 adress. This one handle all connections to the v6-Only-Server from v4-only-clients. (It has a sniproxy on it which is working for a lot of websites.)
On my Domain the AAAA-Record is pointing directly to the v6-only-server and the A record is pointing to the v-server.

I set up FreeRDP-Web on the v6-only-server. If i connect from local lan (via domain) everything is working well.
If I connect from "the outside" the login promt is showing up correctly. I type in my username and pw click on connect. The loading bubbles are showing up for a few seconds and the login promt is showing up again.

I set up log-level to debug. In the windows logfiles is just the following message:
Request from IP of Proxy: /empty_on_purpose => 304 Not modified

I hope you could help me to get those amazing tool working.

[egeu5]

Update: I tested the functionality from an external IPv4 and IPv6 network. It isn't working on IPv6 to and that is without the sniproxy just a single apache proxy is still in the setup. But this should make no difference...
It is working on the internal network (over the domain name) but not if I am in a different one. Did I need any additional ports to forward (more than 443 and 80)?