Comment 76 for bug 1733557

I have this problem, and found a thread here:
https://askubuntu.com/questions/584207/cannot-log-in-at-locked-screensaver-must-log-in-with-switch-user?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa

The fix that worked for me was a permission problem on /etc/shadow
$ ls -l /etc/shadow
-rw-r----- 1 root shadow 2202 Jun 23 12:39 /etc/shadow

For me the permission was:
$ ls -l /etc/shadow
-rw-r----- 1 root root 2202 Jun 23 12:39 /etc/shadow

When I changed the group to shadow and restarted the system. I was able to login from the lock screen again. The reason I believe that restarting is required is that the lightdm crashes if it cannot access the shadow file. and once it crashed, you get the message about authentication failure, switch to greeter.

The explanation was that the lockscreen runs in user context, whereas the greeter runs as root, so the shadow group allows lightdm to access it.

I tried the lock screen several times now, and it is solidly working. I did not change the group from shadow to root, so perhaps an update changed the file permission, or a patch changed the password utility. not sure.