I have an update. I've disabled the intel virtualization technology and hd audio. I've changed the top of lower usable Dram from dynamic to 1gb, and PCIE link speed from auto to gen3 for all pcie.
After that I have managed to connect 3 graphics cards.
However, when I put the 4th one (the last graphics card I have) the computer doesn't switch on. I have just a black screen and a b2 code on the motherboard. How can I solve this?
Make sure you have enough PSU power rating watts to supply your 4 GPU's, What are your GPU types? check also the 4th GPU if its connected correctly thru powered riser and your PSU.
much better if you could tell us your PSU power rating and your GPU types
I doubt it is the PSU. I have a SeaSonic - PRIME Gold 1200W 80+ Gold Certified. My graphics cards are Radeon Pro Duo Professional with 32 GB memory. Their consumption is 200 w maximum while mining.
There isn't a power supply issue. I guess it is smth related to the motherboard.
It is not even starting. The monitor just turns black and there is the b2 code on the motherboard.