Yes, groups
I deleted the group jid and group owner's jid in roster.db and after that seg-fault stopped. And yes, it seems I was re-added in the group Receiving millions of messages (and precisely the reason I *hate* groups)

The sql[ite3] code i ran was:
In my roster this was the only contact with number beginning with a 9 .. you would have to enter more detailed filter if needed

Yappari seg-faulted again, likely because of the sheer amount of new messages. But eventually everything returned to normal after all messages were downloaded. I will just block the person (who is not even my friend) and hopefully will not be added to groups again.