I now just tried with the PICkit3 to:
1. Read a good PIC firmware from a good hash board
2. Write a good PIC firmware to a dead hash board

No effect. The dead hash board is still dead.
So it is probably not just "soft bricked".
So I wait for the answer from BitmainWarranty (www.bitmainwarranty.com).

I have previous experience with the PICkit3.