Yes, I've had problems too because the hyperlink points to an old and closed source for the javascript. There are various answers, essentially you just need to point to a different _src (source). Inside the authoring tool in the plug settings you can manually edit the address (so you don't need to edit the files yourself). I think it will partly depend on your firewall settings (at my university cloudfare addresses often require captchas and thus aren't good). Here are some links that worked for me:

https://cdn.jsdelivr.net/npm/mathjax@2.7.5/MathJax.js

This one is supposed to have been decommissioned but works for me:

https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML

This one is supposed to work too, but NOT for me:

https://cdn.rawgit.com/mathjax/MathJax/2.7.1/MathJax.js

 There are others too if you search online.