lightdm crash (once) & thereafter login screen shows "failed to authenticate" - impossible to login

OS: ubuntu 20.04 with lightdm
HW: Raspberry Pi 4B 8GB

History: this system has worked for well over a year without issues.

There was a random crash of the sound system (this has happened before), my normal cure for this is to reboot. Usually it works.

This time, I could not log in after reboot. The lightdm greeting screen seems to want to auto login (I disabled auto login a long time ago) and shows the error message failed to authenticate.

How do I investigate and fix this problem?

Some things I tried:

1. reboot - no impact, may have caused the initial problem
2. apt reinstall lightdm; reboot - no impact
3. commented out kwallet in pam configuration of lightdm - no impact other than a few error messages less in the logs
4. ssh - remote login works, this is my way in (tty also works)

Edit:

5. removed and reinstalled lightdm - no impact other than changing the login splash screen

   apt purge lightdm
   apt update
   apt install lightdm
   dpkg-reconfigure lightdm
   reboot
   

6. created /etc/lightdm/lightdm.conf as below - it did add a 'login' button to the login screen, but there still isn't any box to enter a username or a password, so I can't login.

   [SeatDefaults]
   greeter-show-manual-login = true
   greeter-hide-users = false
   allow-guest = false
   

Note: my system does not have gdm.

Logs & files:

/etc/lightdm/lightdm.conf does not exist:

> ls -alR /etc/lightdm
/etc/lightdm:
total 24
drwxr-xr-x   3 root root  4096 Sep 13 23:38 .
drwxr-xr-x 151 root root 12288 Sep 13 22:26 ..
drwxr-xr-x   2 root root  4096 Feb  7  2020 lightdm.conf.d
-rw-r--r--   1 root root   466 Feb  7  2020 users.conf

/etc/lightdm/lightdm.conf.d:
total 8
drwxr-xr-x 2 root root 4096 Feb  7  2020 .
drwxr-xr-x 3 root root 4096 Sep 13 23:38 ..

/var/log/apport.log shows many repeats of the following lines, one for each retry or reboot:

ERROR: apport (pid 2458) <date/time>: executable: /usr/sbin/lightdm (command line "lightdm --session-child 13 20")
ERROR: apport (pid 2458) <date/time>: is_closing_session(): no DBUS_SESSION_BUS_ADDRESS in environment
ERROR: apport (pid 2458) <date/time>: this executable already crashed 2 times, ignoring

Crash report (around the first reboot, not sure whether it happened before or after my first reboot to try and solve my sound system issue):

> ls -al /var/crash/
total 228
drwxrwsrwt  2 root     whoopsie   4096 Sep 13 17:14 .
drwxr-xr-x 14 root     root       4096 Dec 15  2020 ..
-rw-r-----  1 root     whoopsie 219359 Sep 13 17:14 _usr_sbin_lightdm.0.crash
-rw-r--r--  1 root     whoopsie      0 Sep 13 17:14 _usr_sbin_lightdm.0.upload
-rw-------  1 whoopsie whoopsie     37 Sep 13 17:14 _usr_sbin_lightdm.0.uploaded

/var/log/lightdm/lightdm.log (last few, relevant lines):

[+6.37s] DEBUG: Greeter connected version=1.30.0 api=1 resettable=false
[+7.24s] DEBUG: Greeter start authentication for ubuntu
[+7.24s] DEBUG: Session pid=2455: Started with service 'lightdm', username 'ubuntu'
[+8.71s] DEBUG: Session pid=2455: Terminated with signal 11
[+8.71s] DEBUG: Session: Failed during authentication
[+8.71s] DEBUG: Authenticate result for user ubuntu: Authentication stopped before completion
[+8.71s] DEBUG: Seat seat0: Session stopped

I just yesterday had an issuethat lightdm would not log me ion anymore but that was on 18.04. what I did to resolve thisd was:

# apt purge lightdm
# apt update
# apt install lightdm
# dpkg-reconfigure lightdm
# reboot

Which fixed it for me, good luck!