This morning, I found the open_chat MUC on igniterealtime to be unresponsive. These were basically the same characteristics as I experienced in OF-983. I suspected another deadlock situation.
Turns out that there indeed was a deadlock, but it appears to be a different one than the one in OF-983. The timing of both occurrences is so close that there probably is a relation somewhere, but I've thought it sensible to track this one in a new issue.
This is the deadlock:
Believed fixed by reverting code.