Hey guess what, it seems I got it to work! At least I'm now finally in the BIOS for the first time!
So here's what I did:
+ plugged all RAM sticks except 4GB in A1: no change
+ plugged in an old graphics card: no change
+ disabled on-board VGA through jumper: no change
+ unplugged graphics card and re-enabled VGA: no change
+ updated BIOS with version 3.3 which Supermicro support gave me
access to: no change
+ updated BIOS to previous version 3.4: apparently no change
My next step would have been to disable the on-board network cards. So
I looked up where the jumper for that is while the board was still
powered after the BIOS update. When I was about to unplug the power,
I heared it beeping, so I looked and it had rebooted. It didn't get
stuck at B2 and booted from an USB stick I had plugged in when I
unsuccessfully followed the BIOS recovery guide before I updated to
BIOS version 3.4 (pressing Ctrl+Home had no effect). So I rebooted
and was finally able to get into the BIOS, and I made the settings I
wanted. After saving and rebooting, it still seemed fine, so I put
the memory sticks back in and it works with them as well. Tomorrow
I'll put into my sever
