Description:
------------
When you start a session and provide a form to fill in,
within 30 minutes the session is destroyed although the documentation says it should be 180 minutes and php.ini does contain:
session.cache_limiter = nocache
session.cache_expire = 180

NOTE: Couldn't reproduce this locally on windows, it only happens on a heavily visited productional website.

Reproduce code:
---------------
<?php
session_name('CMSSESSID');
session_start();
header('Date: '.date('D, d M Y H:i:s \G\M\T', time()));
header('Cache-Control: private, pre-check=0, post-check=0, max-age=0');
if (!isset($_SESSION['test']))
{
    echo '<form action="'.$_SELF.'" method="post"><textarea name="form"></textarea><input type="submit"></form>';
}
else
{
    echo 'session is valid and data is posted';
}
?>

Expected result:
----------------
After waiting for 30 minutes to post the data:

session is valid and data is posted

Actual result:
--------------
The formfield