Lock screen does not unlock with correct password Gnome and Ubuntu 14.04

I had to reinstall recently. I installed normal Ubuntu 14.04. I then installed gnome/gnome shell/gdm. I then logged in my user to gnome. Since then I have been configuring my system.

Yesterday, unlock was working fine. Today, the automatic lock screen is not accepting my password as valid. It just takes the input like it's processing it, then takes me right back to the same password prompt. If I click the 'login as a new user' link, it takes me right back to the lock screen. Ctrl+Alt+F1, etc, does nothing.

I had to resort to a hard boot. I've tried this more then once. (first time was from automatic, idle time lock, second time was from my triggering the lock screen with the keyboard command).

I'm certain I have my password right, as I am able to get logged in after the cold boot.

Solution 1:

Neither of the other two answers solved the problem for me, but some additional searching led to this bug report:

sudo chown root:shadow /sbin/unix_chkpwd
sudo chmod 2755 /sbin/unix_chkpwd

which solved the issue for me.

Solution 2:

I could resolve this issue with the following permission changes:

sudo chown root:shadow /etc/gshadow
sudo chown root:shadow /etc/gshadow-
sudo chown root:shadow /etc/shadow
sudo chown root:shadow /etc/shadow-

Found here: http://ubuntuforums.org/showthread.php?t=1006366

Check also this question: Lock screen - password incorrect