In order to HotSync my PIM data I had to export the data and reimport to a new user account. That account would HotSync correctly. The old account, for my ancient Palm III was trying to drag some games and other stuff to my new N810, and that seemed to make the Sync fail every time. With clean data only, the Sync worked fine.

This was suggested on the Garnet FAQ page from Palm, and it worked for me.