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After updating from 14.10 to 15.04, the login screen is black. Just by changing the mouse pointer do I find the password text field; password typed in and then all works normal.
I had the same problem, Logging in the terminal mode (Alt+F1) and running the command:
sudo dpkg-reconfigure -a
and rebooting fixed my problem.
Install another greeter, like gdm, then run the command sudo dpkg-reconfigure lightdm to switch to it.
sudo dpkg-reconfigure lightdm
Required, but never shown