Comment 93 for bug 1311316

We also have this problem on several machines, and I hope I can contribute to getting it fixed.
Removing light-locker fixed the "double unlock" problem, but not this problem. The machine is a fully up-to-date Ubuntu 14.04 with LDAP authentication.

The output of ls -l /sbin/unix_chkpwd is
-rwxr-sr-x 1 root shadow 35536 Jan 31 2014 /sbin/unix_chkpwd
I cannot post full auth.log because of personal data,
but I will try to remember to grap the last lines of the log the next time the problem appears.

The problem occurs only randomly and not very often (maybe once per week). It happens less often now that the "double unlock" problem is fixed, I guess simply because I have to enter my password less often now.

A reliable work around is to switch to a text console, login as the same user, and run "unity --replace".
This restores the password input field. After unlocking, the panels are missing but I can fix this by opening a terminal window and running "unity --replace" there.