Ten minutes later, it is a little different.
Before (by mistake) I had the output contactor switched on. With it set to remote, and the gun voltage at 20V, wire feed at 350 ipm, it welds fine. When I increase the voltage to 24V (machine is on), the instant the wire hits the work piece, the breaker pops. I tried this twice; 20V is fine, 24V pops the breaker.

Frustrating situation. Maybe time to pick a repair shop.