Quoting myself:
Thanks to Valentin's config the board is booting. Now the main issue are the Intel X550 NICs that will go into firmware-recovery mode after some time and traffic. I am looking into that now.
After some playing around, it seems the Ethernet is now stable. I am not sure what action is responsible for this outcome. It could be that just a power cut was responsible (because I flashed the image with the BMC, the board never went actually completely power-off).
Again, I would appreciate any feedback from people who also have the board (-TF or -CTF) working if they encountered any trouble with the X550.
-Andreas