After updating from UCS 4.2 to UCS 4.3-0 the database mariadb does not start any longer.
Before upgrading make sure the package univention-mysql is installed. When installed it will smoothly migrate from MySQL to mariadb.
You should revert to an snapshot, if possible. Once reverted, install univention-mysql by
If there is no snapshot you might want to remove any mysql and myriadb packages with
apt. Once cleaned, re-install with
Note: Option 2 is untested and you might loosing data this way.