The screen goes black when I lock the system. LightDM Greeter is not displayed, but I can still type my password and unlock the system. Recently I found a workaround for this is to switch to the TTY of the desktop session ((Ctrl+Alt+F7 on my system), which makes LightDM Greeter to appear.
Why do I need to explicitly switch to TTY7 before login screen is displayed? Can login screen be displayed automatically?