Mac Pro (Mid-2012) - no video and a very weird mainboard/processor board issue

[Guest Valdaquendë]

I have a Mid-2012 cMP (5,1, A1289 - 2629) that has a very strange problem (or maybe I just don't have enough information). It powers up but won't boot or drive the display. I have followed the Apple Service Manual's diagnostic procedures for this device but cannot narrow the problem down further than "backplane and/or processor board". I have already tried swapping out the GPU, power supply and backplane, so far to no avail. I hope that someone can enlighten me or suggest further steps...

 

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