I have Ubuntu 14.04 installed with the Lubuntu desktop on a computer being used mainly for netflix (Netflix now works on chrome). In the middle of movies the screen would go blank and revert to lock screen when I pressed the spacebar.
After some troubleshooting, I've determined I have the known and yet unfixed bug where the PowerManager will not run.
I followed the directions above to deal with Lightlocker. The settings above had no effect. I determined I'm also having a second known bug where the settings in Lightlocker to disable screen locking are not working properly (see here https://bugs.launchpad.net/ubuntu/+source/light-locker/+bug/1287255). What worked for me was (Menu>Preferences>Lightlocker) flip switch to disable Lightlocker altogether.