We're facing a similar symptom where emails to a particular list won't go out and seem to be stuck in the out queue. We also noticed that mailman is hitting /hyperkitty/api/mailman/urls infinitely.
We've also seen what I think is the same issue.
We had three messages for the same list being constantly requeued, with the following hyperkitty URL being hit multiple times per second for each message:
GET /mailman3/hyperkitty/api/mailman/urls?mlist=...&msgid=...
The messages were constantly re-appearing in the "out" queue with different file names (I assume they were going via the retry queue).
There are no corresponding log entries in mailman log files, even after turning the default log level up to debug, and no log entries in our MTA log files.
After multiple restarts trying to get some debug information, the hard loop seems to have stopped recurring, and the messages are sat in the "shunt" queue. They've not been delivered to recipients, but they are in the hyperkitty archive.
Any suggestions for how we can debug this further? At the moment this appears to only be affecting this one list.
thanks,
Paul