We have exactly the same setup, and exactly the same problem.
The only temporary solution that I have found is to manually close the network connection in Windows (see, e.g., http://forums.retrospect.com/index.php?/topic/151634-hangups-when-client-computer-disconnects/). Then it is possible to manually stop the execution. This is of course unacceptable, especially since the hanged execution blocks the access to the backup set until the problem is taken care of manually.