Reposting my reply from a few days ago to a thread with the same panic message.

I found this in my logs when some mails that got deleted on the phone were still visible in roundcube. I currently have three users that trigger the panic. One user with an iPad accounts for nearly all of the panics.

The problem shows when the account is accessed from android (thunderbird) or ipad. The same accounts show no problems when beeing used with roundcube or thunderbird on windows.

I enabled core dumps on 2026-01-14 and have ~8500 core dumps of the imap binary until now.

What causes this error and how can I fix it? In the logs all I see is the users login, then the panic.

doveconf -n

2.4.2-2+debian13 (0962ed2104): /etc/dovecot/dovecot.conf

Pigeonhole version 2.4.2-2+debian13 (767418c3)

OS: Linux 6.12.0-custom.kvm x86_64 Debian 13.3 ext4

2026-01-13T21:44:32.317609+01:00 uschi dovecot: imap-login: Logged in: user=<user@example.com>, method=PLAIN, rip=<IP1>, lip=<IP2>, mpid=4194077, TLS, session=<cpkID0tId7dUc9rl> 2026-01-13T21:44:38.455379+01:00 uschi dovecot: imap(user@example.com)<4194077><cpkID0tId7dUc9rl>: Panic: file imap-sieve-storage.c: line 216 (imap_sieve_add_mailbox_event): assertion failed: (ismt->src_box == NULL || ismt->src_box == src_box) 2026-01-13T21:44:38.458755+01:00 uschi dovecot: imap(user@example.com)<4194077><cpkID0tId7dUc9rl>: Error: Raw backtrace: #1 [unw_get_proc_name() failed: -10] -> #2 [unw_get_proc_name() failed: -10] -> #3 i_panic[0x7f6b6c46c2f6] -> #4 [unw_get_proc_name() failed: -10] -> #5 [unw_get_proc_name() failed: -10] -> #6 [unw_get_proc_name() failed: -10] -> #7 mailbox_save_alloc[0x7f6b6ca103c0] -> #8 [unw_get_proc_name() failed: -10] -> #9 command_exec[0x55654e677400] -> #10 [unw_get_proc_name() failed: -10] -> #11 [unw_get_proc_name() failed: -10] -> #12 [unw_get_proc_name() failed: -10] -> #13 client_handle_input[0x55654e67ddc0] -> #14 client_continue_pending_input[0x55654e67e0b0] -> #15 io_loop_call_io[0x7f6b6c565490] -> #16 io_loop_handler_run_internal[0x7f6b6c567230] -> #17 io_loop_handler_run[0x7f6b6c5673c0] -> #18 io_loop_run[0x7f6b6c5675a0] -> #19 master_service_run[0x7f6b6c4b27f0] -> #20 main[0x55654e6688a0] -> #21 [unw_get_proc_name() failed: -10] -> #22 __libc_start_main[0x7f6b6c234ce0] -> #23 _start[0x55654e668eb0] 2026-01-13T21:44:38.459536+01:00 uschi dovecot: imap(user@example.com)<4194077><cpkID0tId7dUc9rl>: Fatal: master: service(imap): child 4194077 killed with signal 6 (core dumps disabled - https://dovecot.org/bugreport.html#coredumps)

coredumpctl dump

        PID: 1071230 (imap)
        UID: 5000 (vmail)
        GID: 5000 (vmail)
     Signal: 6 (ABRT)
  Timestamp: Fri 2026-01-16 19:17:15 CET (2s ago)

Command Line: $'dovecot/imap [user@example.com <IP redacted> UID MOVE]' Executable: /usr/lib/dovecot/imap Control Group: /system.slice/dovecot.service Unit: dovecot.service Slice: system.slice Boot ID: 3668d179bda340229a780d5d43e426c3 Machine ID: 05698baa30ae984c9bd014566e16da18 Hostname: uschi Storage: /var/lib/systemd/coredump/core.imap.5000.3668d179bda340229a780d5d43e426c3.1071230.1768587435000000.zst (present) Size on Disk: 1.1M Message: Process 1071230 (imap) of user 5000 dumped core.

             Module libgcc_s.so.1 from deb gcc-14-14.2.0-19.amd64
             Module libstdc++.so.6 from deb gcc-14-14.2.0-19.amd64
             Module libzstd.so.1 from deb libzstd-1.5.7+dfsg-1.amd64
             Stack trace of thread 1071230:
             #0  0x00007fbd1ca9f95c n/a (libc.so.6 + 0x9495c)
             #1  0x00007fbd1ca4acc2 raise (libc.so.6 + 0x3fcc2)
             #2  0x00007fbd1ca334ac abort (libc.so.6 + 0x284ac)
             #3  0x00007fbd1cc6caa0 n/a (libdovecot.so.0 + 0x6caa0)
             #4  0x00007fbd1cd49e67 n/a (libdovecot.so.0 + 0x149e67)
             #5  0x00007fbd1cc6c3d0 i_panic (libdovecot.so.0 + 

0x6c3d0) #6 0x00007fbd1c42f942 n/a (lib95_imap_sieve_plugin.so + 0x4942) #7 0x00007fbd1c431223 n/a (lib95_imap_sieve_plugin.so + 0x6223) #8 0x00007fbd1d258f10 n/a (libdovecot-storage.so.0 + 0x11bf10) #9 0x00007fbd1d1a843b mailbox_save_alloc (libdovecot-storage.so.0 + 0x6b43b) #10 0x00005590b7ed4a69 n/a (n/a + 0x0) ELF object binary architecture: AMD x86-64

-- Christian Kivalo