No idea if it will be related or not, usually if you "yum update" php, then session directories will have incorrect permissions. But issabel use a custom session directory in recent versions... in any case, you can try something like
chown -R asterisk.asterisk /var/lib/php/session
chown -R asterisk.asterisk /var/lib/php/session-asterisk
Its a long long shot, but won't harm trying it out