When this happens you should actually be able to press Esc to recover the greeter. You can also press Ctrl+Alt+F1 and log in there, following by "sudo service lightdm restart", that will restart the display manager.
That said, can you please clear /var/log/lightdm and ~/.cache/upstart/, try logging in to the unity8 session and collect the logs from both directories and attach here?
When this happens you should actually be able to press Esc to recover the greeter. You can also press Ctrl+Alt+F1 and log in there, following by "sudo service lightdm restart", that will restart the display manager.
That said, can you please clear /var/log/lightdm and ~/.cache/upstart/, try logging in to the unity8 session and collect the logs from both directories and attach here?