It looks like when you first tried to add the user, MariaDB wasn't running, hence the failure you note first.

Did a new vm deploy, no updates installed/ no manual changes done. Besides the username or maybe related, there are some others errors see bold text:

Command used:  journalctl -xeu mariadb.service
[...]
InnoDB: Starting shutdown...
[ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
[...]
[ERROR] Unknown/unsupported storage engine: InnoDB
[ERROR] Aborting

Hmm, that is interesting - and unexpected! That's the same info report originally by Daniele - which I couldn't reproduce. And if that's before making any changes, that suggests perhaps something else/more than what I thought was going on.

I'll try again and explicitly check the journal. And I'll follow your lead and even if I can't exactly reproduce it, I'll try your other steps too.