Hi,
I’m seeing the same issue after upgrading to Dovecot 2.4.2. I reproduced it with both the Alpine Linux build and the official Dovecot Docker image referenced at 1.
In my case, the crash correlates strongly with iOS devices auto-deleting older messages from the Trash folder. As Christian described, it starts after a larger number of IMAP MOVE operations.
Key finding: when I disable the imap_sieve extensions, the crashes stop. Personal Sieve rules continue to work as expected, so the problem appears to be specific to imap_sieve (or its interaction with frequent MOVE operations).
sieve_plugins { sieve_imapsieve = no }
protocol imap { mail_plugins { imap_sieve = no imap_filter_sieve = no } }
My configuration with the enabled imap_sieve extension is located at 2.
Kind regards
Jeff