Dovecot IMAP crashes on a Plesk server with Ubuntu 24.04: Panic: file imap-sieve-storage.c: line 216 (imap_sieve_add_mailbox_event): assertion failed

Symptoms

• Plesk Obsidian is running on Ubuntu 24.04.
• IMAP connections are unstable. Mail clients (e.g. Apple Mail) constantly resync or fail to move emails between folders.
• Dovecot IMAP processes crash repeatedly.

• The following error message appears in /var/log/mail.err or journalctl:

  CONFIG_TEXT: Panic: file imap-sieve-storage.c: line 216 (imap_sieve_add_mailbox_event): assertion failed: (ismt->src_box == NULL || ismt->src_box == src_box)
  Fatal: master: service(imap): child 12345 killed with signal 6 (core dumped)

Cause

Product issue PPPM-15312.
The issue is caused by an incompatibility between Dovecot 2.4.2 (shipped with Plesk Obsidian 18.0.76 on Ubuntu 24.04) and the imap_sieve plugin used by Plesk Email Security.

Resolution

Apply the workaround by configuring the mail_attachment_detection_options parameter:

1. Connect to the server via SSH.

2. Create a custom configuration file to apply the fix:

   # echo 'mail_attachment_detection_options =' > /etc/dovecot/conf.d/999_fix_crash.conf

3. Restart the Dovecot service:

   # systemctl restart dovecot