Comment 35 for bug 1205384

A workaround is to do this on terminal:

sudo apt-get install xscreensaver
sudo wget --directory-prefix=/usr/local/bin/ https://launchpadlibrarian.net/154739229/lxlock
sudo chmod a+x /usr/local/bin/lxlock

You may also want to start xscreensaver as an automatically started application, but that is not necessary, when using the above.