The second possibility I see is the PCIE power port on the motherboard. Should I power both of them with molex ?
As long as you use risers with external power supply - no, no need to power them
The third possibility I see is a bad BIOS setting
There are no special settings for this setup to work, only handy things like automatically power on system after power outage
But it's not working, the GPU isn't detected. But if I swap the Molex to 6 pins on the GPU and the 8 pins to 6 pins on the riser, it works
So don't know why it's enough to power the GPU but not the riser ?
Do you have a multimeter? The best way to find out is to check voltage for a start
No, I don't have multimeter. Will try without the molex converter to see. May it will work with 8 pins to 6 pins. (But no enough 8 pins to 6pins slots to power all the 5 GPU + Risers)