Quickbooks Error Code (120): Authorization Failure: AuthorizationFailure: Unknown Error during Authentication, statusCode 500
Duplicate Document Number Error AuthorizationFailure : XXX Accountant user was deleted while still connected to the company.
This occurs whenever the user who initially set up the Syngency to Quickbooks Online sync is disabled in Quickbooks.
Disconnect the sync by Revoking, then set up the sync with an enabled user.
If this does not resolve the issue, contact Syngency Support