James,
Some times if the load on the client (which you are trying to backup) is high and if that gets delayed because of the load (network load) it may fail with error 53.
This is basically a delayed response from the client, you can try triggering the backup in a diff timing and check the backup.
In my experience the delay caused because of the host name resolution hence it had failed with 53 and the second weired thing which saw is ...
One of the linux client was always failing with 53 and some times 54 ... the issue was with packet reformatting, which means bprd will go back to client with specific request and it waits till 60sec if the request is not fulfil then the backup will fail.
That may not be applicable to your issue but it is related to 53 :)
By the way is it only one client which is failing or multiple ?
Thanks,
Raghava.
