I did RMA the GPU and instantly got a new one from the shop. Good service!
I connected the new GPU with a new riser and all went well. So probably was the GPU.
Sending the risers in for RMA wasn't a good idea or possible since bought overseas...

The stuff underneath the risers always comes loose because of the heat in the (open) rig:-(
Doesn't anybody else have this problem? So all of my risers are 'naked but I'm really careful not to short anything. And this was not what caused my problem.