In this thread (clicky) I temporarily solved my inability to transfer routes from Basecamp 4.6.2 to an RV660. The temporary solution was a Master Reset. However, I found that after the initial successful transfer of routes from Basecamp to the GPS subsequent attempts failed. A new Master Reset enables the transfer success again, and then fails shortly.
At this point, I've given up on transferring routes; perhaps a solution will become apparent later.