Most likely the System Writer is not present any longer. Unfortunately, this is 
a known Windows issue.
You can verify that running "vssadmin list writers" - the 'System Writer' will 
be missing.

Solutions: 
 - For Windows 2003 clients the only method is a reboot.

 - For Windows 2008 clients, run these commands as administrator: 
      CD c:\windows\system32
      Takeown /f %windir%\winsxs\filemaps\* /a 
      icacls %windir%\winsxs\filemaps\*.* /grant "NT AUTHORITY\SYSTEM:(RX)"
      icacls %windir%\winsxs\filemaps\*.* /grant "NT 
Service\trustedinstaller:(F)"
      icacls %windir%\winsxs\filemaps\*.* /grant BUILTIN\Users:(RX)
      net stop cryptsvc
      net start cryptsvc

      Also see  
http://social.technet.microsoft.com/Forums/en-US/windowsbackup/thread/bb760434-de80-4cb6-a9e-bd253e4b037c

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