If a torrent application has been running and torrents start showing a status of ‘unregistered torrent’ a likely cause is that the server IP address has changed. To fix this you need to close the application and flush the DNS, before starting the application again.

To flush the DNS cache:

OS X Leopard / Snow Leopard

$ sudo dscacheutil -flushcache

Windows XP

$ lookupd -flushcache