A couple things I need to ask....
-Did you measure 50V with all the load turned off or only with the panel loaded? (Thinking backfeed from another phase via a 208V piece of equiptment)
He shut the A/C breaker off, now the lights in the bathroom went out completely! I told him I'd come check it out.
-Did you measure phase to phase? (Thinking this might be a corner grounded delta system.. One phase would typically show 0V to ground)
Just off the info given, I'd offer a loose or corroded connection in the disconnect. possibly where a lineside lug attaches to the lineside contacts on C phase OR possibly at the utility test bypass. Without a load, I'm assuming you might get a normal voltage read, but resistance from a bad connection is dropping your C phase voltage. What brand was the disconnect? I'm assuming 80A fuses were installed.
