Using any account, when I click the "save" button to submit changes in my user preferences, I get
Request: POST http://www.mediawiki.org/wiki/Special:Preferences, from 10.64.0.102 via cp1065 frontend ([10.64.0.102]:80), Varnish XID 2174772612 Forwarded for: <My-IP-redacted>, 10.64.0.102 Error: 403, Noise at Mon, 21 Sep 2015 23:37:28 GMT
I tested with 3 accounts at 4 wikis in 2 browsers, trying to change/save a different preference each time.
Same results everywhere.