Samuel,
Interesting! I checked on a PRO board, and the driver IC heatsinks are NOT electrically connected to any pin that I could find, and I checked using the Ohm scale, so even if it was 10M Ohm, I’d have seen something on the meter.
Nevertheless, something blew somewhere.
What board do you have, so I can better direct you to some things to check.
If you plug it in and quickly check the voltage at the FAN connector, what do you get as a reading? (it SHOULD be 24 VDC).
Did you double-and-triple check that nothing dropped on the board and that nothing has ‘stuck’ to the bottom of the board causing any shorts?
BrianV