I tried to move a site yesterday and got this error “cannot manually move an item to drop off library”. Then I deactivated the Content Organizer feature of the destination parent site. Still I got “Operation to Move ‘/parentsite/sonsite/grandchildsite’ to ‘/parentsite/sonsite/grandchildsite/grandchildsite’ failed”. I have no idea what was going on. Then I re-activated Content Organizer feature of the parent site and deactivated it again. When I tried to move the site again, it succeeded.