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You may see an error similar to the following in your Image or File and Folder backup VSS log

Volume Shadow Copy Service error: 
Unexpected error calling routine ConvertStringSidToSid(S-1-5-21-1687195420-861800275-1737445057-500.bak)
hr = 0x80070539,  The security ID structure is invalid.


This error is caused by a stale user profile registry entry that could not be resolved by the VSS Shadow Copy Optimization Writer.


  1. Open the Registry editor 'Regedit.exe'.
    Press the Windows key and type 'regedit'

     
  2. Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList and delete any sub-keys that end in '.bak'
  3. Restart your PC