In theory you can walk through the contents of /etc/pam.d and compare against the output from /var/log/auth.log to see what is going on.
In case you're not aware, each file in pam.d is a potential entry point to ask pam if you can get authority. In your case lightdm. The log entries are fairly self explanatory as far as figuring out which lines in the log come from which lines in the pam file.
According to the docs I found you should be able to add 'debug' to lines in pam.d files to get extra info in the log.
In my setup, I'm using kde, and kdm and I get lots of lines containing (kdm:auth) when I lock my screen and attempt to unlock it (with the wrong password), but nothing when it unlocks successfully. The is next to no comparison between pam.d/kdm and pam.d/lightdm which makes no sense to me, so maybe you can try swapping things around to see if the issue is in the lightdm pam module.
The only other thought I had, is whether you have interesting symbols or characters in your password. If the lightdm lock screen box, isn't coded properly you might find it's not sending what you type to the back end. Try changing your password to something basic (like 1234) to see if it works, if it does, then (change your password back obviously, but) it probably means there is nothing wrong with your pam configuration at least.
Sorry if this doesn't help much, beyond looking at adding pam_debug.so to various pam files (see http://manpages.ubuntu.com/manpages/hardy/man8/pam_debug.8.html), to see what happens, I'm not sure what else to suggest.
Best Answer
Tried many workarounds to no avail. The way I solved this issue was by removing Light-Locker completely and installing gnome screensaver.
Replace Light-Locker with Gnome Screensaver:
light-locker
, then unchecklight-locker
andlight-locker-settings
and click [Apply changes].gnome-screensaver
and check the box to the left and click apply changes again.Or, using the terminal:
It works perfectly, didn't mess up my configuration with
lightdm
, and looks good too. If you're not satisfied with the background shown every time you lock your screen, you can installgnome-tweak-tool
. If you can't find it in the applications menu, you can either run it through terminalgnome-tweak-tool
or or create a launcher using the same command. Once it's open go to the desktop tab, and choose the location of the background of your liking.Now you can lock your screen and enjoy a nice screensaver/locking application that works! Good luck! :)