Troubleshooting/debugging PCB
Don't flex your boards.  A cracked solder joint on a rigid part is possible.  Look for rigid parts with the along axis aligned across the "bow" of the flex.

There is very little chance you will catch this visually since the fracture is just a tiny crack, and with the board returned to its normal shape it probably mostly closed up such that it will be very hard to see.

The easiest fix is probably to run the whole board thru a reflow oven if you have access to one.  Barring that, going around and re-melting all the solder joints might work, but will be tedious.  The cheapest solution is to buy another motherboard, and then not abuse it this time.