Just to keep you all in the loop, the Celeron theory was tested

i bought i3-4 Core CPU, same result. i suspect the B250 Chipset and how the PCI Lanes are address is part of the Problem. Expensive Lesson for us, as we have to buy now Server ontop of the cases we got initially.
(we event tried two other different Mining mainboards, but they had also the B250 Chipsets and did not work out.)
We even were going as far as getting an Octominer Case, but that did not work either. Only working solition so far was a professional Server System.