How to fix MariaDB when it gets stuck during shutdown (Waiting for page_cleaner)?

After a multiple hour long struggle, I could finally resolve the issue.

The culprit was a time setting (timedatectl set-local-rtc 1) for dual booting with Win10.

Due to a recent time change the clock was messed up. (The status log entries in sudo systemctl status mariadb were 1h in the future)

Setting

timedatectl set-local-rtc 0

and rebooting resolved it.