SIP module stops accepting requests after some time

From time to time my Asterisk ( and 11.2.1) all of a sudden stops accepting SIP requests - existing calls go on, but the SIP requests of new calls will time out. Doing a “module reload” does not help, the only thing that helps is a complete restart. The log file (log level debug) does not show any error.
The error occurs once every 2-10 days.

The problems started after updating from 1.8.17 to I’ve tried updating to 11.2.1 but that has not helped either.

Any tips on what could be causing this or how to troubleshoot?

Rebuild with thread debugging enabled (there is a significant performance cost, although the throughput is still reasonably good) and nooptimise (relatively little penalty). When things go wrong, issue “core show locks”. You may need to do this from a new CLI session.

Also use tcpdump, etc., to confirm the requests are actually reaching Asterisk.