Please note that the session cookie name on WMF wikis has changed from
<wiki>_session to <wiki>Session. That is, for example, enwiki_session
to enwikiSession.
This was done in response to a security issue -- we had to find some
way to rapidly cause all session IDs to be regenerated. Changing the
cookie name was the simplest way to achieve that. More details should
become available once we are sure that the security issue is fixed.
-- Tim Starling