Hmmm,. try the absolute minimal system.

Unplug all other boards & connections from the cpu to other boards, this includes the power driver board that plugs onto the MPU.

The only things left connected should be the main power supply and the MPU board...

Powerup and see if you get the MPU LEDs flash (or not).

If no flashing, then you have MPU problem.
If the LEDs do flash, then re-connect the other boards 1-at-a-time (poweroff between each reconnection) and note which connection or board then makes the MPU LEDs stop flashing to find the culprit.

MM