Hi Aki,
To verify whether it's caused by antispam plugin, you could copy/move mail to Spam folder and see if it crashes? That's inconclusive: I can copy stuff in and out of my Spam folder just fine using mutt. I've never seen the crash for my user so far, which suggests this is specific to one IMAP client (or otherwise dependent on circumstances).
I'll ask the user who did have the crash to move some mail, to see if that triggers it again.
Yesterday, the crash would happen repeatedly in long bursts, on connections from the same user, which were probably retries on a failing mail copy. I disabled the antispam plugin for a while, which showed some non-crashing connections from this same user (presumably completing the copy). After re-enabling anti-spam, I've not seen the crash again for this user, probably because there were no pending problematic copies anymore.
Gr.
Matthijs