The user must have delete rights in the folder they are moving it out of and they need to have add/rename rights in the state that it's in to be able to move a file, otherwise it throws an error that for some reason shows up like that. I just reproduced on my system when I didn't have add/rename on the state and it threw the same unique filename error until I turned off unique filenames on my vault and it told me the correct error.
You nailed it on the head. I just figured that out then I read your post. There should really be a difference here. I'm ok with Manufactured prints to be moved in the system when released but not deleted. That's a double edged sword on that one.