Plesk Backup fails intermittently

[tjordan]

The Plesk backup fails occasionally. One night is fine, then the next night it fails. When it does fail it fails following the same domain. The backup log appears to be fine and it closes out gracefully as if there was no error.

The tail of the backup log looks like this:
Start: Save domain 'domainname.com'
Start: Save physical hosting
Exec 'C:\Program Files\SWsoft\Plesk\admin\bin\websrvmng' '--get-error-docs "--vhost-name=domainname.com"'
Exec 'C:\Program Files\SWsoft\Plesk\admin\bin\websrvmng' '--list-vdirs "--vhost-name=domainname.com"'
Exec 'C:\Program Files\SWsoft\Plesk\admin\bin\websrvmng' '--get-vhost-performance "--vhost-name=domainname.com"'
End: Save physical hosting
Start: Save subdomain 'webtr'
Exec 'C:\Program Files\SWsoft\Plesk\admin\bin\websrvmng' '--get-subdomain-performance "--vhost-name=domainname.com" "--subdomain=webtr"'
Exec 'C:\Program Files\SWsoft\Plesk\admin\bin\websrvmng' '--get-error-docs "--vhost-name=webtr.domainname.com"'
Exec 'C:\Program Files\SWsoft\Plesk\admin\bin\websrvmng' '--list-vdirs "--vhost-name=webtr.domainname.com"'
End: Save subdomain 'webtr'
End: Save domain domainname.com'
End: Save client 'Cient Name'
End: Saving...
Start: Starting Plesk control panel.
End: Starting Plesk control panel.
Start: Starting services of domains
End: Starting services of domains

As administrator I get the email notification:
User: HOSTNAME\Plesk Administrator
Running task: C:\Program Files\SWsoft\Plesk\admin\bin\backup.exe --config-file "C:\Program Files\SWsoft\Plesk\Backup\\backup_schedule.psc"
Started: Mon Oct 08 03:00:00 2007
The task output is attached to the e-mail Ended with code 2: Mon Oct 08 03:01:19 2007

And attached to it is the following text file:
Backup error. Unexpected error

Some nights the backup runs fine. Others nights it fails.