I’m reporting this issue on behalf of user A1000:
Since last week, A1000 is unable to log in on Commons. The error message is:
There seems to be a problem with your login session; this action has been canceled as a precaution against session hijacking. Please resubmit the form.
He’s getting the same error message when trying to log in on mediawiki.org to report the problem here on Phabricator. He’s still able to edit on dewiki, probably because he did not log out since the problem first occurred.