Plesk 9.5 backups failing

[PaddingtonC]

I recently upgraded from Plesk 8.6 to 9.5 and have been trying to run Plesk backups from the backup Manager but they are failing with the comment "This file is not a valid backup file" Its however shows the backup to be the correct size but the red dot with the white cross sign on status gives " This is not valid backup. Data cannot be restored from this file"

I am running windows 2003 standard edition.