Comment 1 for bug 1273077

The keyboard shortcut is trying to use xscreensaver which is not installed and should be using lxlock.

~/.config/openbox/lubuntu-rc.xml

Currently:

    <!-- Lock the screen on Ctrl + Alt + l-->
    <keybind key="C-A-l">
      <action name="Execute">
        <command>xscreensaver-command -lock</command>
      </action>
    </keybind>

Change to:

    <!-- Lock the screen on Ctrl + Alt + l-->
    <keybind key="C-A-l">
      <action name="Execute">
        <command>lxlock</command>
      </action>
    </keybind>