Originally Posted by sparvu
I do have currently a single card, the Quadro 4000, slot 0
I guess you men the blue slot closest to the CPU, named "PCIe 2.0 x16_1 slot"?
The MB manual says that this can be x8 or x16, depending on the slot occupation
of the rest of the system. "PCIe 2.0 x16_5 slot" (the blue slot most away from
the CPU) is an unconditional x16 slot, so you may like to try out that one.