The Spec Code (engraved on the metal surface of the processor) should indicate the stepping/revision.
SLB8W <=> E0
Furthermore, did you try a clean installation of Windows* 8.1?
Are you able to install and boot any other operating system?
Yes, it's SLB8W.
Clean installation is not possible at the moment because I need the installed version with many apps and data.
I got a hint meanwhile why the system is not running: http://www.asus.com/support/FAQ/103696/
And I was told that I need a mainboard with Intel 4 series chipset.
Can you confirm that?
I can only provide a list of compatible Intel® Desktop Boards; however, the FAQ only indicates that some features will not work properly.
Do you have any spare drive you could use for a clean installation of Windows* 8.1?
I'll get a ASUS P5Q-VM in the next days and test it with this board then.
I'll let you know the results.