Erreur avec session_start() sous Free

Warning: session_start() [function.session-start]: open(/mnt/151/sdb/d/b/troc.hec/sessions/sess_c63aa5f1ad2c65640123735b65bd9c56, O_RDWR) failed: No such file or directory (2) in /mnt/151/sdb/d/b/troc.hec/forum/index.php on line 2

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at /mnt/151/sdb/d/b/troc.hec/forum/index.php:2) in /mnt/151/sdb/d/b/troc.hec/forum/index.php on line 2

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /mnt/151/sdb/d/b/troc.hec/forum/index.php:2) in /mnt/151/sdb/d/b/troc.hec/forum/index.php on line 2

<?php
session_start();
$bbcode = false;

// On donne ensuite un titre à la page, puis on appelle notre fichier debut.php
$titre = "Index du forum";
include("includes/identifiants.php");
mysql_connect($adresse, $nom, $motdepasse);
mysql_select_db($database);
include("includes/fonctions.php");
autoconnect();
whosonline();
include("includes/debut.php");