Tried to remove the HBA-card, network card and three RAM sticks and boot it again, no difference. Tried a different RAM-stick, no difference. Moved to a different RAM-slot no difference. Have not yet tried to reseat the cables or CPU since that is a bigger job but will try it.
Seems like it flip flops between complaining about core 6 or 7, not sure if that is relevant.
Tried to install TrueNAS 13 on a different machine and move the drive over, same issue.