did you changed the bios settings?
changing x16 and x1 lane speeds to gen1 and enable 4g decoding?
I will try this tomorrow and see if it helps.
This seemed to have helped some. Atleast I got one 1070 working. So far it only works if the PCE-E riser is connected to the 16X PCI-E slot though..
This particular motherboard does not seem to have an option for "4g decoding".
One mention to someone else who might be faced with this problem: it seems like PCI-E splitters (the cards that split a 1X PCI-E port to 2-4 1X PCI-E ports seem to require the PCI-E port to be sett to AUTO from the bios, so setting it to gen1 won't work).