Good evening, in the course of fixing issues with my upgrade I got redis-server running which failed due to the odd and old kernel I (have to) use, configured and installed by the provider of the (virtual) machine. Marc Ballarin, who obviously knows a lot more about that topic an who reported #863477 and brought the solution on the way, mentioned in his original post (https://github.com/systemd/systemd/issues/6318) that he cannot start mariadb either and had to downgrade systemd. So, wventually, this is maybe an issue with my old (and probably quite patched) kernel (3.2.41-042stab123.9) in combination with systemd.
Regards, Harald