It's a motherboard/processor issue, I got around this by using a port multiplier on one of the x1 slots.
I would try messing around with BIOS first like disabling junk like onboard audio and change pci configurations.
Sadly I dont have a multiplier available, Junks are already disabled in the bios.
The annoying part is that I have dozen of this exact model up and running, but this one doesnt want to work...
Clear CMOS (either use the appropriate jumper, or remove the battery and replace it after ~10 seconds).
Then attempt to boot with ONE card, directly installed into the physical PCIe x16 slot closest to the CPU. Can you get your system to boot with this configuration?
My system can boot with 1 gpu connected via riser on the first PCIE16x and I already did many Clear CMOS but without any results.
If with these informations you still think it's a good idea, I can test it
If you're certain all individual cards and risers are working, try adding them one-by-one, starting with one connected to the physical PCIe x16 slot closest to the CPU (use this one for video, as well).
Assuming your BIOS is currently at default settings, ensure that you have enabled 4G decoding, and have PCIe link speed to GEN1 or AUTO.
Already tested that BUT i've didnt test to connect directly in a gpu, i'll try that