MP 1,1-5,1 4,1 backplane flashing power led after 7 seconds no chime no boot

[webbp]

Likely causes and/or suggestions on how to fix? All other components are working with another 4,1 backplane, so I know the problem is with the backplane.

Symptoms: hard disks and GT120 GPU do receive power and spin up. Then the power led stays lit for about 7 seconds, then starts flashing about twice per second, and there is no boot chime nor boot screen.

PS: I have another post about another non-working 4,1 backplane. This is a different one.

PPS: The two non-working backplanes have different problems. Maybe I can use donor parts from one to fix the other?

 

[webbp]

Update: after cleaning and reseating everything, the power led stays on indefinitely, no flashing. However, there is still no chime and no video. The DIAG button produces green PSU PWROK and amber 5V STBY.