I had this issue and can confirm it was resolved by:
chown root:shadow /sbin/unix_chkpwd chmod 2755 /sbin/unix_chkpwd chown root:shadow /etc/shadow sudo chmod g+r /etc/shadow
Ubuntu version: 13.10 gnome-screensaver version: 3.6.1
Not sure if it makes a difference, but my /home/ is encrypted.
I had this issue and can confirm it was resolved by:
chown root:shadow /sbin/unix_chkpwd
chmod 2755 /sbin/unix_chkpwd
chown root:shadow /etc/shadow
sudo chmod g+r /etc/shadow
Ubuntu version: 13.10
gnome-screensaver version: 3.6.1
Not sure if it makes a difference, but my /home/ is encrypted.