I am not convinced. the 8.4 should still be able to turn the motor over. In fact it was able to do it once already.
Inspect the fuse. Beyond that, you risk voiding the warranty on it if you remove the gearbox to do a wire inspection. If you decide to inspect the wiring, the good thing about the p90 is that only 2 screws hold the gearbox in, and all the wiring is external on the gearbox, so you should be able to see if there is a short somewhere.