Hello, I have the same issue as EpicLPer where the system does not boot. Tried flashing the BIOSes from SMDFlea and the symptoms were, again, the same as EpicLPer's (and now HAWKAR's).
I noticed that the revisions of all three motherboards differ (rev. A, B and C) but I don't know if that means anything.

The model number of this HP Z2 SFF G9 is 5F157EA#ABU and the serial number is CZC325719X....