Hey sgundrum
Ok this is a profile error where it cant load up the direct path to a registry key that contains the profile info, basically it cant load up an old profile that may be deleted
1. Start\Run and type "Regedit.msc"
2. Navigate to
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList
3. Go through the profile list and delete the one that has previously been deleted or shouldn't be there, The ones you search through are highlighted in the picture. DO NOT DELETE THE ONES AT THE TOP OF THE PROFILE LIST
4. Once done you should be able to back up straight away
regards
Craig