The log message is the same as the one I posted originally. There is also a backtrace, which I didn't include in the original message.

Oct 21 14:04:05 imap1 dovecot: lmtp(1797565): Panic: file mail-user.c: line 229 (mail_user_deinit): assertion failed: ((*user)->refcount == 1)
Oct 21 14:04:05 imap1 dovecot: lmtp(1797565): Error: Raw backtrace: /usr/lib/dovecot/libdovecot.so.0(backtrace_append+0x46) [0x7f655cc53d76] -> /usr/lib/dovecot/libdovecot.so.0(backtrace_get+0x22) [0x7f655cc53e92] -> /usr/lib/dovecot/libdovecot.so.0(+0x109c6b) [0x7f655cc60c6b] -> /usr/lib/dovecot/libdovecot.so.0(+0x109d07) [0x7f655cc60d07] -> /usr/lib/dovecot/libdovecot.so.0(+0x5cf53) [0x7f655cbb3f53] -> /usr/lib/dovecot/libdovecot-storage.so.0(+0x483c5) [0x7f655cd753c5] -> dovecot/lmtp [10.20.32.185 DATA](lmtp_local_data+0x3e6) [0x564ff3232996] -> dovecot/lmtp [10.20.32.185 DATA](client_default_cmd_data+0x191) [0x564ff32314c1] -> dovecot/lmtp [10.20.32.185 DATA](cmd_data_continue+0x180) [0x564ff3231250] -> /usr/lib/dovecot/libdovecot.so.0(+0x7c656) [0x7f655cbd3656] -> /usr/lib/dovecot/libdovecot.so.0(io_loop_call_io+0x6d) [0x7f655cc76f1d] -> /usr/lib/dovecot/libdovecot.so.0(io_loop_handler_run_internal+0x13a) [0x7f655cc7861a] -> /usr/lib/dovecot/libdovecot.so.0(io_loop_handler_run+0x54) [0x7f655cc76fc4] -> /usr/lib/dovecot/libdovecot.so.0(io_loop_run+0x40) [0x7f655cc77180] -> /usr/lib/dovecot/libdovecot.so.0(master_service_run+0x17) [0x7f655cbe96d7] -> dovecot/lmtp [10.20.32.185 DATA](main+0x225) [0x564ff322fec5] -> /lib/x86_64-linux-gnu/libc.so.6(+0x29d90) [0x7f655c951d90] -> /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x80) [0x7f655c951e40] -> dovecot/lmtp [10.20.32.185 DATA](_start+0x25) [0x564ff322ffc5]

These are the dpkg logs. I have installed the packages manually because due to network policy reasons the Dovecot server cannot access external repositories.

2022-10-21 12:47:32 upgrade dovecot-core:amd64 1:2.3.19.1+dfsg1-2ubuntu2 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:32 status half-configured dovecot-core:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:32 status unpacked dovecot-core:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:32 status half-installed dovecot-core:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:32 status triggers-pending man-db:amd64 2.10.2-1
2022-10-21 12:47:32 status unpacked dovecot-core:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:32 upgrade dovecot-imapd:amd64 1:2.3.19.1+dfsg1-2ubuntu2 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:32 status half-configured dovecot-imapd:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:32 status unpacked dovecot-imapd:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:32 status half-installed dovecot-imapd:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:32 status triggers-pending ufw:all 0.36.1-4build1
2022-10-21 12:47:32 status unpacked dovecot-imapd:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:32 upgrade dovecot-lmtpd:amd64 1:2.3.19.1+dfsg1-2ubuntu2 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:32 status half-configured dovecot-lmtpd:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:32 status unpacked dovecot-lmtpd:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:32 status half-installed dovecot-lmtpd:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:32 status unpacked dovecot-lmtpd:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:33 upgrade dovecot-managesieved:amd64 1:2.3.19.1+dfsg1-2ubuntu2 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:33 status half-configured dovecot-managesieved:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:33 status unpacked dovecot-managesieved:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:33 status half-installed dovecot-managesieved:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:33 status unpacked dovecot-managesieved:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:33 upgrade dovecot-mysql:amd64 1:2.3.19.1+dfsg1-2ubuntu2 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:33 status half-configured dovecot-mysql:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:33 status unpacked dovecot-mysql:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:33 status half-installed dovecot-mysql:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:33 status unpacked dovecot-mysql:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:33 upgrade dovecot-pop3d:amd64 1:2.3.19.1+dfsg1-2ubuntu2 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:33 status half-configured dovecot-pop3d:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:33 status unpacked dovecot-pop3d:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:33 status half-installed dovecot-pop3d:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:33 status unpacked dovecot-pop3d:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:33 upgrade dovecot-sieve:amd64 1:2.3.19.1+dfsg1-2ubuntu2 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:33 status half-configured dovecot-sieve:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:33 status unpacked dovecot-sieve:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:33 status half-installed dovecot-sieve:amd64 1:2.3.19.1+dfsg1-2ubuntu2
2022-10-21 12:47:33 status unpacked dovecot-sieve:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:33 configure dovecot-core:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:33 status half-configured dovecot-core:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:56 status installed dovecot-core:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:56 configure dovecot-imapd:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:56 status triggers-pending dovecot-core:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:56 status half-configured dovecot-imapd:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:56 status installed dovecot-imapd:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:56 configure dovecot-lmtpd:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:56 status half-configured dovecot-lmtpd:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:56 status installed dovecot-lmtpd:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:56 configure dovecot-mysql:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:56 status half-configured dovecot-mysql:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:56 status installed dovecot-mysql:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:56 configure dovecot-pop3d:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:56 status half-configured dovecot-pop3d:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:57 status installed dovecot-pop3d:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:57 configure dovecot-sieve:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:57 status half-configured dovecot-sieve:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:57 status installed dovecot-sieve:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:57 configure dovecot-managesieved:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:57 status half-configured dovecot-managesieved:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1
2022-10-21 12:47:58 status installed dovecot-managesieved:amd64 1:2.3.16+dfsg1-3ubuntu2.2~ppa1