Many years ago, I had a defective backplane which had prevented a computer from working for good.
But I wasn't aware from the start, that the problem had been caused by the backplane.
Does your backplane look like this ?
Attachment:
backplane.png
If yes, plug the PCBs into a new backplane, and see what happens.
;---
Nice green solder resist on the backplane PCB, that yes, but no copper inside the holes.
When soldering the connectors into the backplane, _then_ mounting the connectors with screws to the PCB,
this causes mechanical stress to the pads where the connector pins are soldered.
Could happen, that some of the PCB traces "go lose" from the pads after a while...
...when an old school floppy drive decides to send vibrations through the whole computer, for instance.
Would be good to know for sure that the malfunction isn't caused by "a simple mechanical problem" before digging deeper into the hardware.
It looks exactly like my backplane. Interesting. I tried with a different backplane, but it did not start at all. Maybe a PSU issue on that one. Will give it another try. Thanks.