crawler stops indexing results

[noerw]

One of the these issues regularly occurs shortly after starting a new crawl:

1. The crawler keeps crawling, but only indexes the initial seed URLs: Outlinks are fetched, the crawlstatus index is still updated, but no new pages appear in the results index.

2. The crawler stops crawling entirely after a short time: No pages are fetched anymore at all, though the crawlstatus index contains newly discovered pages.

I can't reproduce this every time. Restarting the crawler continues the crawl as expected without further issues.
It seems to be a caching issue, as this only occurs for a crawl whose exact configuration has been used before (?).
After a thorough search I could not identify the cause; reverting to earlier versions does not fix the issue!