Problems with $session variable -- every Page generates a new SessionID

[Ralf]

Hello everybody,

I have a problem with a PW installation that i can't quite understand or i don't know where to look for forgotten settings?

I have a separate member area in the frontend. When I click from page to page, I always get a new SessionID assigned. 
Until now this was not a problem, but now I wanted to store form data in a session variable and use it over several pages, but this is not possible, because it gets a new SessionID assigned every time I call another page?

In the files I only found the following values:

config.php

/*
 * Your PHP has a configuration error with regard to sessions.
 * It is configured to never clean up old session files. Please correct this by adding the following to your /site/config.php file
 * 
 */
 ini_set('session.gc_probability', 1);

and in every php file for a template I have this code in the first line:

<?
//This page is only displayed if a member is logged in, otherwise the user is redirected to the login mask.
if($user->isLoggedin() != true) $session->redirect($config->urls->root.'access/login/', false);

.....

Otherwise I would not be aware of any other code that could be responsible for such behavior ?

Is there any other setting for this topic in the backend or elsewhere?
For tips I would like to thank you already now

Thanks Ralf