Sounds like you have tried everything already, so it would be a hardware escalation: 1-800-821-2797 or 1-800-835-6100.
When I attempt to scan a document using my Workcentre 6515, I receive a 005-121 error code indicating there is a jam in the document feeder. I have checked all components of the DADF and cannot find a jam or anything that looks out of place. I have also tried unplugging the sensor.