ASROCK H81 Pro BTC, supposed to work with 6 GPUs.
PCI Gen was set to 1, also tried Gen2 before I went to bed, but the driver fails there aswell.
I have not seen any PCI settings regarding the x16, x8 or x4 speed.
I*ll check the DDU later today.
Have been trying Ubuntu, but the amdgpu-pro driver fails to install due to driver signature verification or other funny stuff.