It's probably using one of the cards as primary (monitor output) and not the onboard GPU. Try plugging the monitor into a card or the onboard graphics port.
I plugged the monitor hdmi cable in the GPU instead of the motherboard and it worked! Thank you so much, I could've found this solution on my own, but with your help the problem got solved in less than 10min, many thanks again!
I set Gen1 in the mobo settings for PCIe speed, is there anything else I should set in the mobo bios settings?