It seems it is a Ubuntu bug.
https://bugzilla.gnome.org/show_bug.cgi?id=620555
** Changed in: gnome-screensaver (Ubuntu)
Status: Invalid => New
** Bug watch added: GNOME Bug Tracker #620555
https://bugzilla.gnome.org/show_bug.cgi?id=620555
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Can't unlock screen when using a password-less account
https://bugs.launchpad.net/bugs/589299
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