Bad SATA backplane in Ultra 4

I purchased a "for parts or repair" Ultra 4 with expectations (and appropriate price) that it was for parts. Seller reported it showed "no disks present" but I found that slot 4 worked fine. A drive in slot 1 spun up but failed diagnostics and 3 & 4 didn't even spin up. There are clear signs this was once dropped (which I was not told, but no matter). I took an old NVX and started by swapping the power supply, then the SATA backplane. Low and behold, it works great with the replacement backplane. But then I got to trying to figure out what is wrong with the backplane and I can't tell. The drives are supported by the chassis, so a fall/drop should not have been able to flex the board enough to fracture traces or vias, I wouldn't think. And all the connectors buzz out fine (all the pins on slot 4 that buzz to something on the backplane to motherboard connector do so on all connectors).

Thinking something might have just been loose, I put the original back in and the problem returned.

Now, I've successfully upgraded an NVX to an Ultra4 (which, unlike an NVX, can run OS6) for a song, so I shouldn't be complaining. But does anyone have any ideas what could be wrong with the backplane? It would be nice to have both going. I'm sure finding a schematic is out of the question.

Anyone have an old non-working 4-slot unit (NVX, Ultra4, and Ultra 4Plus are definitely compatible, I suspect Pro4 and maybe even NV+ V1) from which they'd like to sell the backplane cheap? Can anyone confirm if an NV+ backplane is compatible?