Hi there,
I can give the "solution".
The risers I have seem to be really instable. After plenty of different tries, I get the GPU starting on Riser with the PCIe x16 port.
So, no config problem or else... Just bad hardware quality I think.
I spent hours on this :-/
Glad you find a solution yourself good to hear, also make sure you buy the best riser out there which I believe it's the 009 versions, if you check the riser board closely you should see a number like that on it, we have 005, 006, 007, but the 009 works best, more stable than others