ok maybe a new setting that error.log is disabled by default 
could this be the issue?
https://archlinuxarm.org/forum/viewtopic.php?f=65&t=14690
though it comes back to the same as the previous link & ‘setenv mesontimer “0”’ followed by a reboot didn’t seem to help
did you guys tried the solution from the Odroid forum post?
yes - edited /boot/boot.ini to switch off mesontimer (setenv mesontimer “0”) & rebooted - 
maybe it’ll work for fhals…
nigelf , Heureka, you found it!
Edited my boot.ini and set
setenv mesontimer “0”
According to boot.ini this setting should disturb a smooth video playback.
After rebooting, Mariadb is back to work on my Odroid.
(Lighttpd quit working now, hopefully, this is another story)
Well done, thanks!
great news - I shall have another go later. Fingers crossed!
fhals
maybe open a new post if there are further issues on other apps. Just to avoid to many issue in a single post.
Yes, I will if I can’t get it back to work on myself.
Seems to be plugin-related, not mesontimer.
Thanks.
btw indeed MariaDB has errorg.log disabled by default now. I did a test installation on Buster and error.log is enabled
Good to know that behavior changed using Bullseye 
…& I can confirm that the mesontimer solution worked here too - i had simply forgotten to relink the databases
All my installations are up and running, upgrade to Bullseye is successfully finished. Thank you all for your time and knowledge!