I Installed Lubuntu 18 on Virtual Box 5.2.22.
Changed Display Resolution in Preferences -> Monitor Settings.
Then I restarted Lubuntu.
Lubuntu login menu displayed, but after login there is only the black screen.
If I do nothing for some time and the system is locked, login menu is displayed.
So I can see only the login menu and after login there is the black screen.
How to fix that?