Unlock dialog does not appear occasionally in Ubuntu

[Elazar Leibovich]

This is the second ubuntu installation I'm having this problem with.

Occasionaly, after the screen is locked, the Unlock dialog does not appear,
and thus it is impossible to log in. I see the cursor and the desktop
wallpaper, mouse is fully functional, but I cannot login.

I resort to `service lightdm restart`, but is there a more elegant
workaround?
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