For your checks, I would suggest to use a different pin and see if the BLtouch works as intended like:
M558 P9 C"^exp.4" 5 F60 T1000
And connect the white pin to the EXP-Pin 4.
Before, make sure that your BLtouch is giving out 0 and 3.3/5V on the white pin (check that with a multimeter). Just to be sure, the BLtouch has no weird short, that is damaging your board.
I'm pretty sure, that the BLtouch will give out 5V if its powered via 5V.
The duet exp pins are to my knowledge not 5V tolerant (only 0-3.3V), so you might need to put a level shifter or voltage divider in between.
As I cant find a documentation (nor here https://duet3d.dozuki.com/Wiki/Connecting_a_Z_probe neither there https://duet3d.dozuki.com/Wiki/Duet_Wiring_Diagrams) of how the Z_probe_in pin is implemented, I cant give you further help on that.
I hope this helps you to check and workaround this issue.
