Installing an RSS reader extension has broken user login.
Only on the commercial paid-for hosting mind, not my own crappy in-house box, which works.
This is A Known Bug (FFS!) See, "There seems to be a problem with your login session; this action has been canceled as a precaution against session hijacking."
If tempfile space starts to fail for some reason, MediaWiki login fails. It gets its cookies in a knot (Why?!), and fails with the exquisitely incomprehensible error message above. This may be triggered by writing temp files (such as, for instance, caching RSS feeds.
This is why PHP coders shouldn't be allowed nice things.