update: FINAL SOLUTION: after bios downgrade, and OS install, tried another bios upgrade. See post below.
Fixed with a bios downgrade!
Now the system is able to run Ubuntu and Unraid again.
I found a similar issue in this post (a similar Unraid + Ubuntu boot problem) .. that got fixed with a bios-downgrade.
Different vendor though.
My Shuttle motherboard came shipped with the latest BIOS "03/10/2022 16:38:39 Ver: SW580000 1.04") and had no such problems listed.
But went ahead and downloaded the previous 1.01 BIOS , thats all, just wanted to share and save you the worrying, i already did that.
