As you are using a third party plugin, please check with the expert as you can see it works fine at our demo. There must be something conflicting with the plugin. 

I have renamed this and moved it to the correct area for third party product issues.

It most likely isn't in the core as you show it works on the demo. If it were an issue with the SEPHP script, it normally wouldn't work on the demo. As third party products can and do modify source, disabling the plugin you have would not have removed the modifications. You said you are using a pagespeed module. You'll need to check with that expert to see if there is a conflict. If not, then you need to troubleshoot by checking your error logs and perhaps in the browser inspect element but the first place to check is via the expert who made your plugin since you can see it works at the demo.