I have configured my screensavers to lock the desktop after some time; and sometimes, e.g. when I leave my desk, I prefer to lock the screen myself using the title bar "Lock/Switch account..." feature.

While trying to log in again, I enter my password, but the password is labeled as "invalid".

As a workaround, I have to use the mouse to go to the "Switch User..." menu in the title bar, click it, and wait for that other login page to appear, which is quite similar to the screensaver-lock page. (It also lists other the usernames to choose from)

There I enter the same password, and it gets accepted, I'm logged in, unity desktop appears.

Login at the console also works.

Any idea how to diagnose and solve the problem?

Linux xxx 3.19.0-28-generic #30-Ubuntu SMP Mon Aug 31 15:52:51 UTC
2015 x86_64 x86_64 x86_64 GNU/Linux

unity 7.3.2

Compiz 0.9.12.1

There seems to be nothing of interest in kern.log and syslog, but here is something from /var/log/auth.log

    Sep 17 17:20:29 xxx lightdm: pam_kwallet(lightdm-greeter:setcred): pam_sm_setcred

Sep 17 17:20:29 xxx lightdm: pam_unix(lightdm-greeter:session): session opened for user lightdm by (uid=0)

Sep 17 17:20:29 xxx systemd-logind[843]: New session c13 of user lightdm.

Sep 17 17:20:29 xxx lightdm: pam_ck_connector(lightdm-greeter:session): nox11 mode, ignoring PAM_TTY :2

Sep 17 17:20:29 xxx lightdm: pam_kwallet(lightdm-greeter:session): pam_sm_open_session

Sep 17 17:20:29 xxx lightdm: pam_kwallet(lightdm-greeter:session): pam_kwallet: open_session called without kwallet_key

Sep 17 17:20:30 xxx lightdm: pam_succeed_if(lightdm:auth): requirement "user ingroup nopasswdlogin" not met by user "knb"

Sep 17 17:20:33 xxx CRON[37168]: pam_unix(cron:session): session closed for user munin

Sep 17 17:21:10 xxx lightdm: pam_kwallet(lightdm:auth): pam_sm_authenticate

Sep 17 17:21:10 xxx lightdm: pam_kwallet(lightdm:setcred): pam_sm_setcred

Sep 17 17:21:10 xxx lightdm: pam_unix(lightdm-greeter:session): session closed for user lightdm

Sep 17 17:21:10 xxx lightdm: pam_kwallet(lightdm-greeter:session): pam_sm_close_session

Sep 17 17:21:10 xxx lightdm: pam_kwallet(lightdm-greeter:setcred): pam_sm_setcred

Here are some pictures of the screens I have to go through:

Here I have unsuccessfully typed in my regular password. It contains only ascii characters.

Switch user ... (choose my own account, I don't need to switch to another one).

This works.

EDITED: just before end of +150 bounty deadline

I was able to solve this problem myself (after following though all hints and links spread throughout all the ~5 answers so far)

I had to comment out this line in file /etc/pam.d/lightdm:

#auth sufficient pam_succeed_if.so user ingroup nopasswdlogin

I think the reason was that (many months ago, when I was the "only one" with physical access to my computer) I've added myself to the group that can login without password, and autologin to lightdn after boot/reboot. Then some day I changed this back to "login needed after reboot" but for some reason the previous no-login configuration was removed improperly from all the config files.

Now can login again :-)

A note on the bounty / "grading":

The first answerer was closest to the solution by saying something like "look closely at what's in /etc/pam.d". The answer was also the longest and the most thorough.
However I checked all the other answers as valuable, that's all I can do for now, I think.