Lock screen doesn't work

[Norbert_X]

See

21.10 screen dim problem

It is a side-effect of bugs in mate-screensaver and mate-menus: Please subscribe to both and mark yourself as affected. Possible workaround is to remove MATE Screensaver and temporary replace it by XScreenSaver: sudo apt-get purge --autoremove mate-screensaver sudo apt-get install xscreensaver systemctl --user enable xscreensaver.service systemctl --user start xscreensaver.service then test it using Ctrl+Alt+L. Reverting to default screensaver is possible by systemctl --user disable …

for solution.