Devo 12S Thold error

Hopefully someone can help out with a Devo 12S I have for repair that is throwing a 'Thold' error on booting up that cannot be cleared. The tx cannot be booted into update mode either, so no way of flashing Deviation onto it.

The hold switch has been replaced, albeit minus the PCB board that is usually attached. I am wondering is there was a resistor built into the board that would need one of correct value fitting to the replacement switch?
I have traced connections and there are no breaks.

Any help will be greatly appreciated. Thank you in advance!

I am guessing you could disconnect all the switches except the trainer switch and see if you can boot into the programming mode.

Thanks for the reply. Have previously tried that, but unfortunately it still fails to allow the tx to open up for flashing.

Thanks vlad. I had previously checked for bridging on the MCU, and have reflowed it just to be doubly sure. Inspected with 40x magnifier and nothing bridged. Error persists.
Anywhere else causing it, bar the MCU being shot, seems unlikely?