Light locker is blank after locking the screen on Xubuntu 18.04

[katsar0v]

I have a ThinkPad T480s, but tried on t470s after a fresh install of Xubuntu 18.04:

(base) katsarov@katsarov-ThinkPad-T470s:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.2 LTS
Release:    18.04
Codename:   bionic

Here is output from service lightdm status:

(base) katsarov@katsarov-ThinkPad-T470s:~$ service lightdm status
● lightdm.service - Light Display Manager
   Loaded: loaded (/lib/systemd/system/lightdm.service; indirect; vendor preset: enabled)
   Active: active (running) since Tue 2019-07-30 14:46:09 CEST; 54min ago
     Docs: man:lightdm(1)
 Main PID: 1060 (lightdm)
    Tasks: 7 (limit: 4915)
   CGroup: /system.slice/lightdm.service
           ├─1060 /usr/sbin/lightdm
           └─1704 /usr/lib/xorg/Xorg -core :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch

Jul 30 15:37:49 katsarov-ThinkPad-T470s lightdm[11907]: PAM adding faulty module: pam_kwallet.so
Jul 30 15:37:49 katsarov-ThinkPad-T470s lightdm[11907]: PAM unable to dlopen(pam_kwallet5.so): /lib/security/pam_kwallet5.so: cannot open shared object file: No such file or directory
Jul 30 15:37:49 katsarov-ThinkPad-T470s lightdm[11907]: PAM adding faulty module: pam_kwallet5.so
Jul 30 15:37:49 katsarov-ThinkPad-T470s lightdm[11907]: pam_succeed_if(lightdm:auth): requirement "user ingroup nopasswdlogin" not met by user "katsarov"
Jul 30 15:37:52 katsarov-ThinkPad-T470s lightdm[11907]: pam_unix(lightdm:auth): authentication failure; logname= uid=0 euid=0 tty=:1 ruser= rhost=  user=katsarov
Jul 30 15:37:54 katsarov-ThinkPad-T470s lightdm[11909]: PAM unable to dlopen(pam_kwallet.so): /lib/security/pam_kwallet.so: cannot open shared object file: No such file or directory
Jul 30 15:37:54 katsarov-ThinkPad-T470s lightdm[11909]: PAM adding faulty module: pam_kwallet.so
Jul 30 15:37:54 katsarov-ThinkPad-T470s lightdm[11909]: PAM unable to dlopen(pam_kwallet5.so): /lib/security/pam_kwallet5.so: cannot open shared object file: No such file or directory
Jul 30 15:37:54 katsarov-ThinkPad-T470s lightdm[11909]: PAM adding faulty module: pam_kwallet5.so
Jul 30 15:37:54 katsarov-ThinkPad-T470s lightdm[11909]: pam_succeed_if(lightdm:auth): requirement "user ingroup nopasswdlogin" not met by user "katsarov"

Here are lightdm logs after locking the screen:

[+3098.07s] DEBUG: Seat seat0: Locking
[+3098.07s] DEBUG: Seat seat0: Creating greeter session
[+3098.07s] DEBUG: Seat seat0: Creating display server of type x
[+3098.07s] DEBUG: Using VT 8
[+3098.07s] DEBUG: Seat seat0: Starting local X display on VT 8
[+3098.07s] DEBUG: XServer 1: Logging to /var/log/lightdm/x-1.log
[+3098.07s] DEBUG: XServer 1: Writing X server authority to /var/run/lightdm/root/:1
[+3098.07s] DEBUG: XServer 1: Launching X Server
[+3098.07s] DEBUG: Launching process 11811: /usr/bin/X -core :1 -seat seat0 -auth /var/run/lightdm/root/:1 -nolisten tcp vt8 -novtswitch
[+3098.08s] DEBUG: XServer 1: Waiting for ready signal from X server :1
[+3098.41s] DEBUG: Seat seat0 changes active session to 
[+3100.55s] DEBUG: Got signal 10 from process 11811
[+3100.55s] DEBUG: XServer 1: Got signal from X server :1
[+3100.55s] DEBUG: XServer 1: Connecting to XServer :1
[+3100.61s] DEBUG: Seat seat0: Display server ready, starting session authentication
[+3100.61s] DEBUG: Session pid=11818: Started with service 'lightdm-greeter', username 'lightdm'
[+3100.63s] DEBUG: Session pid=11818: Authentication complete with return value 0: Success
[+3100.63s] DEBUG: Seat seat0: Session authenticated, running command
[+3100.63s] DEBUG: Launching process 11821: xubuntu-numlockx
[+3100.73s] DEBUG: Process 11821 exited with return value 0
[+3100.73s] DEBUG: Seat seat0: Exit status of xubuntu-numlockx: 0
[+3100.73s] DEBUG: Session pid=11818: Running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/lightdm-gtk-greeter
[+3100.73s] DEBUG: Creating shared data directory /var/lib/lightdm-data/lightdm
[+3100.73s] DEBUG: Session pid=11818: Logging to /var/log/lightdm/seat0-greeter.log
[+3100.77s] DEBUG: Activating VT 8
[+3100.77s] DEBUG: Locking login1 session c4
[+3100.77s] DEBUG: Activating login1 session c9
[+3100.77s] DEBUG: Seat seat0 changes active session to c9
[+3100.77s] DEBUG: Session c9 is already active
[+3100.81s] DEBUG: Greeter connected version=1.26.0 api=1 resettable=false
[+3101.39s] DEBUG: Greeter start authentication for katsarov
[+3101.39s] DEBUG: Session pid=11907: Started with service 'lightdm', username 'katsarov'
[+3101.40s] DEBUG: Session pid=11907: Got 1 message(s) from PAM
[+3101.40s] DEBUG: Prompt greeter with 1 message(s)
[+3104.47s] DEBUG: Continue authentication
[+3106.42s] DEBUG: Session pid=11907: Authentication complete with return value 7: Authentication failure
[+3106.42s] DEBUG: Authenticate result for user katsarov: Authentication failure
[+3106.42s] DEBUG: Session pid=11907: Exited with return value 1
[+3106.42s] DEBUG: Seat seat0: Session stopped
[+3106.43s] DEBUG: Greeter start authentication for katsarov
[+3106.43s] DEBUG: Session pid=11909: Started with service 'lightdm', username 'katsarov'
[+3106.44s] DEBUG: Session pid=11909: Got 1 message(s) from PAM
[+3106.44s] DEBUG: Prompt greeter with 1 message(s)
[+3112.64s] DEBUG: Continue authentication
[+3112.67s] DEBUG: Session pid=11909: Authentication complete with return value 0: Success
[+3112.67s] DEBUG: Authenticate result for user katsarov: Success
[+3112.67s] DEBUG: User katsarov authorized
[+3112.67s] DEBUG: Greeter sets language en_US
[+3112.70s] DEBUG: Greeter requests session xubuntu
[+3112.70s] DEBUG: Seat seat0: Returning to existing user session katsarov
[+3112.70s] DEBUG: Unlocking login1 session c4
[+3112.70s] DEBUG: Activating VT 7
[+3113.00s] DEBUG: Seat seat0: Stopping greeter
[+3113.00s] DEBUG: Terminating login1 session c9
[+3113.01s] DEBUG: Session pid=11818: Sending SIGTERM
[+3113.01s] DEBUG: Activating login1 session c4
[+3113.01s] DEBUG: Session pid=11909: Exited with return value 0
[+3113.01s] DEBUG: Seat seat0: Session stopped
[+3113.01s] DEBUG: Seat seat0 changes active session to c4
[+3113.01s] DEBUG: Session c4 is already active
[+3113.01s] DEBUG: Greeter closed communication channel
[+3113.01s] DEBUG: Session pid=11818: Exited with return value 0
[+3113.01s] DEBUG: Seat seat0: Session stopped
[+3113.01s] DEBUG: Seat seat0: Stopping display server, no sessions require it
[+3113.01s] DEBUG: Sending signal 15 to process 11811
[+3113.23s] DEBUG: Process 11811 exited with return value 0
[+3113.23s] DEBUG: XServer 1: X server stopped
[+3113.23s] DEBUG: Releasing VT 8
[+3113.23s] DEBUG: XServer 1: Removing X server authority /var/run/lightdm/root/:1
[+3113.23s] DEBUG: Seat seat0: Display server stopped

After locking the screen, the screen is blank.. but if I enter my password, I can log in again and everything comes back to life.

[romanek-adam]

I'm experiencing exactly the same issue on my Dell Latitude E6420 - the screen goes blank after locking or suspending/resuming. It goes back when I type my password properly...

Tested on fresh Xubuntu 18.04.3 with all packages updated as of 2019-12-01.

[jangorecki]

There doesn't seems to be solution for this issue. I posted question on askubuntu, added bounty, no replies for few days already. https://askubuntu.com/questions/1191719/xubuntu-black-screen-after-lock-screen-light-locker-lightdm-logind-xfce4-po I think xubuntu is losing user base due to this issue.

[katsar0v]

I think xubuntu is losing user base due to this issue.

Lost me as well...