Doxgen can handle LaTeX by either rendering it to images and adding them to the HTML output, or by using MathJax. The default is the former; it is disabled if MathJax is enabled.
Our current configuration sets
thereby disabling the default LaTeX support. However, we also set
MATHJAX_RELPATH = http://www.mathjax.org/mathjax
which causes Doxygen to look for MathJax at http://www.mathjax.org/mathjax/MathJax.js, which is a 404.
The correct MATHJAX_RELPATH is https://cdn.mathjax.org/mathjax/latest. Changing to that should fix the problem.