Screen does not lock after inactivity


Ubuntu 21.10 mate.
After leaving my computer a while the screen should be locked. However, it is not. It is light greyish and I can vaguely see what windows are open. I usually go to a terminal, type reboot and I can work again. But its a safety risk of course.

Any tips on howto troubleshoot this?
windowskey-L works fine, so there is a lockscreen working.


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It is known issue, solution see at