Comment 4 for bug 424789

This bug appears to be upstream, it has also been reported in Gentoo;

http://bugs.gentoo.org/show_bug.cgi?id=276583

Switching off session encryption in suhosin bypasses the issue for now. Workaround is to edit;

/etc/php5/apache2/conf.d/suhosin.ini

and add;

suhosin.session.encrypt = off

then restart Apache.