You were installing the riser from the wrong side :/ I didn't know/believe it would be even possible... Did you read the online manual? Page 13 and 14?
If you had problems with 4th screw, did you try to uninstall the IEC14 connector and then screwing the mobo's 4th screw?
You inserted the riser from wrong side. It is supposed to go in from the GPU side not from the motherboard side.
Those cut-outs in the bottom surface are matching the plastic edges on the bottom of the riser's pci-e slot.
Ohhhhh...
That's... embarrassing...