The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "xxx" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition

Le problème est de permuter le chargement de la session et de la classe afin que toutes les classes soient disponibles lorsque le gestionnaire de session dé-serialise les informations.

<?php
session_start();
require_once ("file_to_load.php");

doit devenir :

<?php
require_once ("file_to_load.php");
session_start();