I'm running a server with dovecot that has the users in a mysql database. 

On Leap 42.3, the package dovecot22-backend-mysql handled the database
connection, but this package was uninstalled when zypper dup'ing to Leap 15.1.

Unfortunately its replacement dovecot23-backend-mysql was _not_ installed
during zypper dup. (Wild guess: Maybe you should add an Obsoletes: to it? I'm
not sure if it works, but there's hope ;-)

The obvious workaround is   zypper in dovecot23-backend-mysql   - but ideally
that shouldn't be needed.