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.
- Open the MS Registry editor 'Regedit.exe'.
Press the Windows key and type 'regedit'
- Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList and delete any sub-keys that end in '.bak'
- Restart your PC