For closure, given I've taken up your time with this:
I think I solved the problem, though it's rather embarrassing. The first mobo, that got RMA'd, had this track damage, down by the gfx PCI slot. Well I took out the new board today and found almost identical (but note not identical - its not the same board that got mixed up somehow) scratch.
Can't be sure, but logical explanation is that somehow, after decades of doing this, I've suddenly developed some very sloppy technique for putting in the gfx card that was causing damage. If it is these scratches then its still a bit mysterious, because it took couple of weeks first time for problem to emerge, but second time was immediate. Also that i managed to twice do hardware damage that...