Comment 89 for bug 1733557

I've experienced the same problem on a fully patched 16.04 LTS.
This issue only occurred in the last few weeks & may have corresponded to when I added 3 new users to the machine.
I have previously used the first option in https://askubuntu.com/questions/859581/how-to-change-the-unity-lockscreen-wallpaper to modify my lock screen wallpaper.
After this recent "Authentication failure" issue occurred I recompiled the Greeter Schema as per the link above (sudo glib-compile-schemas /usr/share/glib-2.0/schemas) which has cured the problem on my machine at least.