It appears that the MO TeX does not understand macros \mod and \pmod any more; it used not to be the case. As a result for many questions the typesetting is broken.
MathJax v2.6 was released on Dec 30, close to the OP. These updates can lead to errors if a browser cache contains a mix of MathJax components with different versions. In that case, clearing the browser cache from all things MathJax (and for some browsers, additionally restarting the browser) should help.