Maybe sessions aren't working properly on the site.... possibly a bad session_save_path in the php.ini file.
Try saving a session variable on one page:
<?php
session_start();
$_SESSION['test']= "works";
?>
and then print its value on the next page like:
<?php
session_start();
var_dump($_SESSION['test']);
?>
If session variables aren't working that would explain the error.