From what I have been able to access, there are quite a few possibilities - but after having tried quite a few combinations and been talking to the supplier it begins to emerge that it might be a microcode problem after all.
The thing is, that having tried my 2 Intel Xeon ES (QAMT - Stepping B1) they refused to work, both in Asus Z9PE-D8 WS and SuperMicro X9DR3-LN4F+O (that also, with a combination of 3 different PSU's, 4 different GPU's, 3 different RAM types, this combined with shifting the placement of the VGA/GPU (on some site people claim that that is having an effect of the 5A error - in regards to CPU1 or CPU2 PCIE lines) and trying both 1 and 2 active CPU's.
But, the supplier have successfully gotten some Intel Xeon ES (QA92 - Stepping B0) to work in both the Asus Z9PE-D8 WS and the SuperMicro X9DAi (and has proven it with a CPU-Z screenshot to me (not verified though - but supplier is trustable).
At least for me it must be the microcode difference between the B0 and B1 stepping that could be causing havok (I remember that my older Asus P6T6 could run the i920 C0 stepping on all BIOS'es, but had to upgrade to 0407 to be able to run the D0 stepping). Or maybe I am grasping at straws.
At least if we compile a list with exact stepping information and motherboards revision number/bios we would be able to get a more full overview of what will and will not work.
[edited]
Searching the internet, I have found several claiming to have the Intel Xeon ES work on the Asus Z9PE-D8 motherboard.
Code:
Stepping name ** CPU-Z step ** Stepping codename ** Number observed
*******************************************************************
QA92 ** 2 ** ** 1
QA91 ** 2 ** ** 1
** 2 ** ** 2
QB7R ** 5 ** C0 ** 5
*******************************************************************
No screenshots of CPU-Z or similar seen:
*******************************************************************
QAMT ** ** ** 1
QB7R ** ** ** 1
[/edited]
Best regards