As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
MXP - FLT 717 EN ROUTE MXP TO PHL. AFTER TAKEOFF, EICAS MESSAGE 'ATC FAULT', AND ATC FAULT LIGHT ILLUMINATED WITH THE TRANSPONDER SELECTED IN LEFT AND RIGHT. THE FLIGHT RETURNED TO MXP AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE INSERTED A PIN IN THE CONTROL PANEL CONNECTOR. BITE CHECKED OK.