Problem
After updating from UCS 4.2 to UCS 4.3-0 the database mariadb does not start any longer.
Solution
Before upgrading make sure the package univention-mysql is installed. When installed it will smoothly migrate from MySQL to mariadb.
Option 1
You should revert to an snapshot, if possible. Once reverted, install univention-mysql by univention-install univention-mysql.
Option 2
If there is no snapshot you might want to remove any mysql and myriadb packages with apt. Once cleaned, re-install with univention-install univention-mysql.
Note: Option 2 is untested and you might loosing data this way.