You can look up the Fiddler log files (Help > Open Applications Logs Folder) and investigate any logged errors before the backend message. This kind of error is usually received when you don't have access to one of the FIddler Everywhere API endpoints, so also check if you have full access to the following endpoint:

  https://*.getfiddler.com/
  https://*.amazonaws.com/

If you are unable to resolve the issue, please send us the Fiddler Everywhere log files privately through the support email (Help > Contact Support).