Solved.
It did not set the "correct" math default renderer.
The following lines (see
http://docs.mathjax.org/en/latest/output.html) did the job:
<script type="text/x-mathjax-config">
MathJax.Hub.Register.StartupHook("End Jax",function () {
var BROWSER = MathJax.Hub.Browser;
var canUseMML = (BROWSER.isFirefox && BROWSER.versionAtLeast("1.5")) ||
(BROWSER.isMSIE && BROWSER.hasMathPlayer) ||
(BROWSER.isSafari && BROWSER.versionAtLeast("5.0")) ||
(BROWSER.isOpera && BROWSER.versionAtLeast("9.52") &&
!BROWSER.versionAtLeast("14.0"));
var CONFIG = MathJax.Hub.CombineConfig("MMLorHTML",{
prefer: {
MSIE:"MML", Firefox:"HTML", Opera:"HTML", Chrome:"HTML", Safari:"HTML",
other:"HTML"
}
});