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NSILMike - my google search for "windows backup C:\Windows\System32\config\systemprofile" found this ==>>
Error code 0x8100002F and or error code 0x80070002 when you back up files in Windows 7
I might also add it was the first thing in the results list.
BTW - The libraries that KB article refers to would be Windows libraries not calibre libraries.
IMO - the Windows Backup feature is not fit for purpose, I use Macrium Reflect.
Added : calibre would not write to that folder, but windows might if you moved the calibre configuration folder to C:\Users\
Administrator\AppData\Roaming.
BR