The backup was not performed because and error occurred while copying files to the backup disk.
The problem may be temporary. Try again later to back up. If the problem persists use Disk Utility to repair your backup disk.
I did reboot my ReadyNAS and run a volume scan which found no errors. I know I can't use Disk Utility to repair the ready nas. I also ran a backup on a USB drive that I use for a second back up, and all went well.
I am not sure what to try next.