> what I did to fix the problem was "apt-get purge light-locker".
Here they say that the default greeter (lightdm-gtk-greeter) of light-locker shoud be changed to a different greater, eg slick-greeter https://github.com/the-cavalry/light-locker/issues/114#issuecomment-474709454 https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/1801609/comments/27 So no need to install any other locker like I mentioned above.
