Hello everyone,
I updated to version 12.0.18 Update #41 now. I started to verify all the log files and found this in maillog:
dovecot: imap-login: Login: user=<
[email protected]>, method=PLAIN, rip=::1, lip=::1, mpid=8823, TLS, session=<a1myaZsT4gAAAAAAAAAAAAAAAAAAAAAB>
dovecot: imap-login...