Problem with Synchronous motor
Could it be slipping a pole? That can happen if the torque demand on the load side is greater than what the synchronous motor can deliver. You may want to check the generators or, assuming there is a double hafted gear box, check that for binding.
Also, are the generators immediately or already connected to the synchronous motor when the Korndorfer starter switches into full Run mode? If so, you may want to let the motor torque stabilize before doing so, and maybe stage them one after the other. Hopefully there is a clutch or something to allow the synchronous motor to get to speed before engaging the generators. If not, and it was working before, then go back to what I first said, look for a mechanical problem in the system.