The problem is now solved.
I don’t exactly know how … I guess the main issue occurred in a multi-relay configuration, where one of the relays wasn’t working correctly. I also suspect I made some mistakes in the tests that followed from the problem.
However, I think there is a serious issue in the latest version of DeltaChat. In version 2.56, there is no way to delete a relay immediately. The removed relays stay active for a while (90 days, I assume), and during this period, they continue to advertise TURN servers. This means that if I add a relay and later delete it, I have no way to stop using its TURN server.
Additionally, if an unpublished relay stops working, I will get connection error messages in DeltaChat all the time!
All in all, I believe there should be a way to truly remove a relay and immediately terminate all connections to it. It’s also somewhat annoying to see removed relays still listed in the connections section.