A first known occurrence with Qantas in 2008, then Eva Air in 2012, and was it last year with Lufthansa ... Switch 2 ADRs OFF to regain control.
While working as a designer for an OEM, I recall a similar situation that I was assigned to work on. There were a couple reports of all three of the redundant ADIRU systems used for the autopilot experiencing failure at the same time. The three ADIRU boxes were all mounted next to each other on the same platform in the EE bay. Under certain flight conditions, the platform would experience a structural vibration coupling mode that would cause the ADIRU's to shut down. All three ADIRU's shutting down required the pilot to take control.
The navigation systems allowed the aircraft to operate more efficiently, but not necessarily more safely.