After installing the new windlass, we ran the it without any load to get the forward/reverse connections to the control box right. We ran up and down with no load about half a dozen times, no problem, although we were a bit surprised that the motor noise was quite different in up from down. (as distinct from the ratchet clanking)

A couple of days later, we were about to try it under load, but before putting the chain on the gypsy, tried it again. It runs SLOWLY out and will not run at all in the up direction. The recommended 100 amp breaker popped after several seconds when in the up position. Tried again, same problem.
We took the windlass off the deck and connected direct to the terminals with jumper cables to separate wiring issues from the motor. Breaker popped again, with no motion.

The 3 terminals are all open circuit to the casing. Resistance across terminals is a fraction of an ohm, but I cannot really measure since the digital meter reading jumps around.

Will return to dealer as soon as we can (not easy due to our location)
Does anyone have any ideas on how to fix myself, or further checks?