Found the mistake.
From 2.15 localOverrides.conf I copied accidentally the line
$webworkURLs{MathJax} = "https://cdn.jsdelivr.net/npm/mathjax@2/MathJax.js?config=TeX-MML-AM_HTMLorMML-full";
Changing it to
$webworkURLs{MathJax} = "$webworkURLs{htdocs}/mathjax/es5/tex-chtml.js";
solved the problem.