Checking more in depth, I figured out that the javascript file MathJax.2.7.5.js that tries to be loaded is located under /var/www/discourse/plugins/discourse-math/public/mathjax/MathJax.2.7.5.js, while it looks like it tries to load it from a different location, see pic:
Version: 2.9.0.beta6 | Commit: af3262d70a238f31ab54ad0ccf7dd26fb71a5134