Ciao non sò se ti risolve l'errore, non credo.. cmq prova a riscrivere la pagina cosi:

Codice PHP:
<?php
session_start(); //questo va sempre prima di qualsiasi output..

$host="";
$user="";
$pwd="";

$link=mysql_connect($host,$user,$pwd) or die("Non riesco a connettermi");
mysql_select_db ("");


$Utente = $_POST['user'];  // (ho provato sia con gli apici che senza) usa gli apici
$Pass = $_POST['pwd'];
echo "$_POST['user']
$_POST['pwd']
";

        if (isset($Pass))
          {
          $query_login="SELECT * FROM Persone
                        WHERE User='$Utente'
                        AND Password='md5($Pass)'";
         $result=mysql_query($query_login,$link) or die("Errore di autenticazione. ");
         $num = mysql_num_rows($result);
            if ($num != 0) {
                while ($row = mysql_fetch_array($result))
                {
                    $_SESSION['Username'] = $row['User'];
                    $_SESSION['Usergroup'] = $row['Gruppo'];
                    header('location:pages/index.php');
                    die();
                    }
                }
           else header('location:index.php?Login=Failed');
          }
    
?>
<html> //qui manca il DOCTYPE importantissimo..
<head>
<title>Esempio login</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>

<body >

<div>Login <?php echo "$_GET['Login']"; ?></div>
<form name="form1" method="POST" action="index.php">
  <input type="text" name="user" value="Nome Utente" /> /*brutti i div..*/ 
  <input type="password" name="pwd" value="Password" /> /*tra l'altro vuoti non servono a niente */
  <input type="submit" name="Login" value="Login" />
</form>
</body>
</html>