Finally even me landed on the AjaxHelper for give a bit of coolnes (;)) to an application of mine. I’m still using 1.1.18 version of CakePHP.
The problem finally was that when I did an ajax request call the session was restored. Asking in the Mailing List the problems seems to be that when an ajax call has been made the browser doesn’t send a correct User-Agent.
Starting from the hint gave me in the answer to my post, I found the solution that was setting the CAKE_SECURITY in app core.php to a level of medium or lower.