User blog comment:Kirkburn/Technical Update: September 14, 2015/@comment-6475116-20150915210238/@comment-11733175-20150916084044

Something's actually broken anything using  which is what importScript and importScriptPage expand to before passing the URL onto importScriptURI. The bug seems limited to wikis with ContentReview enabled, but as that includes Dev Wiki it has quite noticeable effects.

It doesn't seem to be related to a hook callback, i.e. something that modifies content before it's output, and I can't spot anything in the code that implemented, so it's a bit confusing.