Error message on roundcube webmail interface: no connection to imap server. Error message on /var/log/mail:
dovecot: imap-login: Fatal: Dovecot version mismatch: Master is v2.2.27, imap-login is v18.104.22.168 (if you don't care, set version_ignore=yes)
Case is solved. The reason for this was etc-update which merged "trivial changes" automatically. I restored /etc/dovecot and /etc/dovecot/conf.d files from backup and now it works again.