Why isn't my javascript working?

So I’ve got two sites that are exact duplicates of each other. The only difference is the location (one is on a dedicated server and one on a separate shared server).

I installed a plugin called WordPress Pop up and it does everything I need it to do. The only problem is that the pop up doesn’t work properly on the live site while it works perfectly on the test site. Am I correct in thinking that, since Javascript/jquery is client side, it will be interpreted exactly the same way regardless of what server the js files are located? If that is the case then I think it might have something to do with different versions of PHP since the plugin files are of course built with PHP (referencing the JS). Anyway the problem is that the pop up does not close when you X out on the live site but it closes on the test server.

See for yourself:

Test site:
https://news-string.com/ca_test/

Live site:
https://www.communityadvocate.com/2012/11/16/marlborough-wildcats-fall-in-state-championship/

Any help would be greatly appreciated. I’ve stared at the problem so long I think I have tunnel vision! It’s amazing that I have trouble-shooted problems infinitely more complicated than ‘this box doesn’t x out’ yet this is the one that I am literally about to give up on.