the https://c328740.ssl.cf1.rackcdn.com/mathjax/2.3-latest/MathJax.js link
doesn't exist anymore. It's seem to be replaced by https://cdn.mathjax.org/mathjax/latest/MathJax.js

if this javascript library is not loaded under ssl environement the Atto editor plugin does not function properly.

Maybe it's a good Idea to change the link in future Moodle source (hardcoded) ?

For site administrators who wants to change the link manually : /admin/settings.php?section=filtersettingmathjaxloader