I don't think this is cached page, because this problem appear more and more
even after changing pages and login/logout operations.
I have try set mediawiki_mw__session expiration time the same as in other
cookies.
So, I have changed includes/GlobalFunctions.php in 2854, where I set first
parameter to $wgCookieExpiration instead of 0
Also i change session name by set $wgSessionName variable in my
LocalSettings.php
I have really not tested, but I think this have fix my problem.