B737-800 Captain reported loss of aircraft control and autopilot disengagement while in cruise. An attempt to reengage the autopilot resulted in the same event. The flight crew returned to departure airport.

Narrative

After take-off on Aircraft X ZZZ1-ZZZ2; at approximately 12000 ft. MSL; the First Officer (FO) engaged autopilot B as it was his leg. Shortly after engaging; the aircraft rolled sharply to the right. Approximately 30 degrees bank in 1 second. The autopilot then disengaged. The FO recovered the aircraft; and when it was stable again; he attempted to re-engage the autopilot again using autopilot B. The aircraft did the same sharp roll to the right. He recovered again; and hand flew. We discussed trying autopilot A; but decided that we would not. We did not want a 3rd repeat of the event as we believed it was extremely uncomfortable to the passengers and Flight Attendants. We then discussed whether we felt we should continue to ZZZ2. Being an ETOPS flight and we had not yet reached the shoreline; neither of us felt we should continue. I thought we should get another opinion; so I called Dispatch via SATCOM and had Dispatch patch us through to the Flight Operations Duty Officer (FODO). The FO flew the airplane and leveled off at FL250. Upon talking to the FODO and explaining our situation; the FODO agreed with us that we should return to ZZZ1 hand flying the aircraft. The FO flew got vectors back to the ZZZZZ arrival to ZZZ1 while I managed the situation. We decided it was normal operation to hand fly to ZZZ1 as there were no adverse reactions from the plane without the autopilot engaged. We landed overweight in ZZZ1 without incident. Our landing weight was approximately 163200 pounds and touchdown less than 500 FPM. Upon returning to the gate; maintenance was there to meet us and ran some tests. They said that FCC B had completely failed and Aileron Actuator 2 had failed. We swapped into a new plane and continued safely to ZZZ2. This event affected safety; but overall; I was happy with how we handled it. I believe we covered all bases before making the decision to divert. Overall; I'm happy with how we handled the situation. We discussed the situation among ourselves and added Dispatch and the FODO into the conversation. We briefed the Flight Attendants and passengers and ultimately made a safe and conservative decision. We then made a safe flight to ZZZ2 in a new aircraft.