Line 598 $session_obj =& singleton::getInstance('php_session');
is simply instantiating an object from the php_session class, so $_SESSION will remain empty until the following call to session_start().
What is the value in $session_name?
What is the value in $_GET['session_id']?
These two values should point to an existing session_id. Is it the correct id? Does that session data still exist? If your session handler is the database (which it should be by default) then look in the php_session table of the AUDIT database.