Since I attached an external monitor to my laptop I'm no longer able to login when resuming from suspend. LightDM appears on both screens and shows my name in the login box, but the password field is not visible and entering the password anyway has no effect.
The machine is still running, and I can access it by SSH or an alternative shell (with
ctrl-alt and function keys), but I can't get back to my previous Unity session.
Logging out and in again without suspending works correctly.
The laptop is a Dell Precision M6700 with an nVidia Quadro K4000M and the external monitor is connected via DisplayPort 1.2. The OS is Ubuntu 16.04 and the problem occurs with both the latest nVidia drivers and the Nouveau display driver.
What can I do to fix LightDM?