Xwindows authentication failure after playing a DVD

I have Ubuntu-Mate, 18.04, which works quite well. I copied a family DVD onto it tried to play it in MythTV; screen went black, "Please wait...." was displayed, and it stopped there. I logged in remotely and tried to kill x (sudo service lightdm stop). The TV screen went black and displayed "Started bpfilter" at the top left corner.

I'm not sure I recall all the steps, but I tried "sudo startx" manually. It partially started and gave this message:

"You must be a member of the "mythtv" group before starting any mythtv applications. Would you like to automatically be added to the group? (Note: sudo access required)"

I answered "No" and it didn't work, so I pressed the reset button. After reboot, x-windows tries to start and displays a password dialogue box. If I enter the correct password, it goes black for a few seconds, then returns back to the same display and dialogue. If I enter an incorrect password, it gives an an "Invalid password..." message and returns to the same dialogue.

Not sure where to go from here, any suggestions?

PS - Seeing these errors in the logs:

From /var/logs:

lightdm.log has this at the end:
...
[+366.97s] DEBUG: Continue authentication
[+368.91s] DEBUG: Session pid=3387: Authentication complete with return value 7: Authentication failure
[+368.91s] DEBUG: Authenticate result for user don: Authentication failure
[+368.91s] DEBUG: Greeter start authentication for don
[+368.91s] DEBUG: Session pid=3387: Sending SIGTERM
[+368.91s] DEBUG: Session pid=3493: Started with service 'lightdm', username 'don'
[+368.91s] DEBUG: Session pid=3387: Exited with return value 1
[+368.91s] DEBUG: Seat seat0: Session stopped
[+368.92s] DEBUG: Session pid=3493: Got 1 message(s) from PAM
[+368.92s] DEBUG: Prompt greeter with 1 message(s)

auth.log has this at the end:
...
Nov 16 14:21:30 johnny systemd-logind[956]: New session c7 of user lightdm.
Nov 16 14:23:34 johnny lightdm: pam_unix(lightdm:auth): authentication failure; logname= uid=0 euid=0 tty=:0 ruser= rhost= user=don
Nov 16 14:23:34 johnny lightdm: pam_winbind(lightdm:auth): getting password (0x00000388)
Nov 16 14:23:34 johnny lightdm: pam_winbind(lightdm:auth): pam_get_item returned a password
Nov 16 14:23:34 johnny lightdm: pam_winbind(lightdm:auth): request wbcLogonUser failed: WBC_ERR_AUTH_ERROR, PAM error: PAM_USER_UNKNOWN (10), NTSTATUS: NT_STATUS_NO_SUCH_USER, Error message was: The sp
Nov 16 14:23:36 johnny lightdm: PAM unable to dlopen(pam_kwallet.so): /lib/security/pam_kwallet.so: cannot open shared object file: No such file or directory
Nov 16 14:23:36 johnny lightdm: PAM adding faulty module: pam_kwallet.so
Nov 16 14:23:36 johnny lightdm: PAM unable to dlopen(pam_kwallet5.so): /lib/security/pam_kwallet5.so: cannot open shared object file: No such file or directory
Nov 16 14:23:36 johnny lightdm: PAM adding faulty module: pam_kwallet5.so
Nov 16 14:23:36 johnny lightdm: pam_succeed_if(lightdm:auth): requirement "user ingroup nopasswdlogin" not met by user "don"
Nov 16 14:25:24 johnny sudo: don : TTY=pts/0 ; PWD=/home/don ; USER=root ; COMMAND=/usr/bin/mc
Nov 16 14:25:24 johnny sudo: pam_unix(sudo:session): session opened for user root by don(uid=0)