Comment 7 for bug 1627304

I could reproduce the issue on Ubuntu 16.10. Additional observations:

* Logging in to a guest session fails also when you do it from the
  login screen without being logged in as a regular user, so I doubt
  that the issue is related to screen locking.

* The zenity warning dialog shows up, but without its title bar - can
  it possibly be compiz related? (The rest of the screen is simply
  black.)