How can I delete a warranty registration? Is there ANY way to add units after registration creation?

I am having trouble deleting warranty registrations. I get the error "You cannot delete a warranty registration while a dependent record is being updated. Please try again later." even if I have no additional records open or being edited. This happens even though there is no case for the warranty registration, and regardless of whether it was generated by WRM or entered manually.

We also are running into a separate error ("You must select a unit that has been associated with the item. You can verify the unit on the item record.") when trying to add customer information to a registration generated by WRM before the unit existed on the item record. As far as I can tell, there isn't a way to update the unit field once the registration has been created, but I would appreciate verification.