Comment 1 for bug 1327902

The old locking system? So you still have xscreensaver installed? We discourage people from having both lockers installed, as it could come to conflicts (and I suppose what you're describing here is one of them).

If "light-locker-command -l" works from the commandline, then this is certainly not a bug in light-locker, but either 1) a bug in xfce4-settings or 2) something in the way you set the shortcut.