Malcolm,
What does the panel say when you check on that particular backup via the backup Tab in the Console? Also, you say it's backed up successfully in the past, have you changed anything on the Client or Router that could have affected the process? Also, is there any indications in the Log files of the Server?
One problem that happened a few times in the past, was that there were failures due to the network card in the Client - either changing the drivers, or changing the settings from Auto to fixed speed sometimes helped.
Colin