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Since I upgraded to Xubuntu 16.04 every time I lock the screen I get a black screen (with a white dash cursor in the top left corner) and there's nothing I can do to unlock it (I've tried Ctrl-Alt-F1 and Ctrl-Alt-F7 but nothing happens), except hard reboot.

Do you have suggestions to solve (or debug) this issue ?

To debug

• Install accountsservice and xserver-xephyr, where:
  
  
  
  
  • 
    accountsservice for Enhanced user accounts handling.
  
  
  • 
    xserver-xephyr for LightDM test mode.
  
  
  
  

Then run LightDM as an X application for debugging: lightdm --test-mode --debug

• LightDM's log file is /var/log/lightdm/lightdm.log, you will need root privilege to view it.


• Or see output of dmesg with sudo dmesg.

List of workaround methods

• 
  

  Try to suspend and resume again

  
  
  

  Press CtrlAltF1,
  then login and type systemctl suspend.
  Press power button to resume and switch to tty7 by
  CtrlAltF7.
  In my case, I did 2 times to escape the blank screen.

  
  



• Use DRI2 instead of DRI3 may solve the problem: https://wiki.archlinux.org/index.php/intel_graphics#DRI3_issues



• 
  

  Disable at-spi-dbus-bus.desktop (may NOT work)

  
  
  

  sudo mv -v -i /etc/xdg/autostart/at-spi-dbus-bus.desktop /etc/xdg/autostart/at-spi-dbus-bus.desktop.disabled

  
  



• Replace light-locker with xscreensaver, then reboot.

References:

• https://wiki.archlinux.org/index.php/LightDM#Testing


• https://bbs.archlinux.org/viewtopic.php?pid=1671675#p1671675


• 
  https://bugs.launchpad.net/ubuntu/+source/light-locker/+bug/1320989 with comment #17


• 
  https://bugs.launchpad.net/ubuntu/+source/unity/+bug/1617471 with comment #25

Well, sudo apt-get install --reinstall xscreensaver may do the work for you. If this doesn't work try sudo apt-get purge --auto-remove xscreensaver && sudo apt-get install xscreensaver.

When I experience this problem, I press ctrl + alt + del, then enter the password again and somehow it fixes it.

On xubuntu 18 I used to get blank screen after locking and pressing keys did not help except

ctrl + alt + F7. On this combination I was seeing something like this session is locked you'll be redirected few seconds. And then I was able to see.

Then I read there How to unlock locked session? and executed this:

sudo apt-get update && sudo apt-get install xscreensaver && sudo apt-get remove light-locker

it did not help.

Then I searched for my screen saver gui settings and disabled screen saver.
Then on locking screen I see this weird xscreensaver window, but its much better than blank screen and need pressing ctlr + alt + F7