There seems to be a long lasting bug in KDE not allowing CTRL+ALT+L to lock the screen. Actually it’s pretty annoying that nothing happens when pressing that short cut. Reading the bug history unveils that this may have been fucked up during a recent upgrade.
The solution is simple – fix the configuration and revoke the short from kscreenlocker.
Navigate to “System Settings – Global Shortcuts”. Select “kscreenlocker” and within the “File” dropdown, choose “Remove component”.
Once the shortcut has been removed, choose “kde session manager” as kde component.
Select “Lock Session” and assign the default “CTRL+ALT+L” shortcut to it again.
Hit “Apply” and have fun again 🙂