Probleme session en ligne

Bonjour,
je fais une administration sur un site, ca marche en local sous ZMWS, mais une fois en ligne sur free, ca marche plus.. j'ai des erreur avant l'ouverture de la sessions, pourtant je n'ai pas d'espace avant, j'ai meme edité mon fichier index et connexion avec un editeur hexa, mais g toujours les memes erreurs ...

script :

<?php include("connexion.php");?>
<?php if(isset($_POST['valider']))
{
$log_ok=false;
if((!empty($_POST['login']))&&(!empty($_POST['pass'])))
{
$sql="
SELECT *
FROM user
WHERE user_login ='".$_POST['login']."'
AND user_password ='".$_POST['pass']."'
";
$result=mysql_query($sql,$connexion);
$row=mysql_fetch_object($result);
if(is_object($row))
{
session_start();
$log_ok=true;
$_SESSION['login']=$_POST['login'];
$_SESSION['pass']=$_POST['pass'];
header("Location: liste.php");
}
}
}
?>

messages d'erreurs :


Warning: session_start() [function.session-start]: open(/mnt/102/sdb/b/1/revel.thibault/sessions/sess_ee636f7e006342b90d7da66baa36ce36, O_RDWR) failed: No such file or directory (2) in /mnt/102/sdb/b/1/revel.thibault/reveillon_electros/index.php on line 17

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at /mnt/102/sdb/b/1/revel.thibault/reveillon_electros/index.php:17) in /mnt/102/sdb/b/1/revel.thibault/reveillon_electros/index.php on line 17

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /mnt/102/sdb/b/1/revel.thibault/reveillon_electros/index.php:17) in /mnt/102/sdb/b/1/revel.thibault/reveillon_electros/index.php on line 17

Warning: Cannot modify header information - headers already sent by (output started at /mnt/102/sdb/b/1/revel.thibault/reveillon_electros/index.php:17) in /mnt/102/sdb/b/1/revel.thibault/reveillon_electros/index.php on line 21