Scene sync problem - master stays in old scene


I have a problem with automatic scene sync. Master player triggers scene change:

public void OnStartButtonClick()
  if (PhotonNetwork.IsMasterClient) {

And then happens something very strange: slave client goes to the correct scene, but master stays in old scene.

Unfortunately it is not easy to reproduce this bug, because it's happening from time to time.

Any idea how can I debug it?