How can I lock screen on lxde
First (in a terminal) start your editor:
vi ~/.config/openbox/lubuntu-rc.xml
Then search for the mentioned faulty code:
<keybind key="C-A-L"> <action name="Execute"> <command>xscreensaver-command -lock</command> </action> </keybind>
and change it to use the dm-tool:
<keybind key="C-A-L"> <action name="Execute"> <command>dm-tool lock</command> </action> </keybind>
Then add a section, so the Windows+L combination works too:
<keybind key="W-L"> <action name="Execute"> <command>dm-tool lock</command> </action> </keybind>
Finally, finish the editor (saving the file) and activate it:
openbox --reconfigure
Good luck