[Resolvido] Erro na $_SESSION

Pessoal, tenho uma rotina de login e quando faço o teste local, funciona certinho, mas quando mando para o servidor da erro, abaixo segue a rotina e o erro:

<?php require_once('Connections/painel_config.php'); ?>
<?php
if (!function_exists("GetSQLValueString")) {
function GetSQLValueString($theValue, $theType, $theDefinedValue = "", $theNotDefinedValue = "")

 if (PHP_VERSION < 6) {
 $theValue = get_magic_quotes_gpc() ? stripslashes($theValue) : $theValue;
 }

 $theValue = function_exists("mysql_real_escape_string") ? mysql_real_escape_string($theValue) : mysql_escape_string($theValue);

 switch ($theType) {
 case "text":
 $theValue = ($theValue != "") ? "'" . $theValue . "'" : "NULL";
 break;
 case "long":
 case "int":
 $theValue = ($theValue != "") ? intval($theValue) : "NULL";
 break;
 case "double":
 $theValue = ($theValue != "") ? doubleval($theValue) : "NULL";
 break;
 case "date":
 $theValue = ($theValue != "") ? "'" . $theValue . "'" : "NULL";
 break;
 case "defined":
 $theValue = ($theValue != "") ? $theDefinedValue : $theNotDefinedValue;
 break;
 }
 return $theValue;
}
}
?>
<?php
if (!isset($_SESSION)) {
 session_start();
}

$loginFormAction = $_SERVER['PHP_SELF'];
if (isset($_GET['accesscheck'])) {
 $_SESSION['PrevUrl'] = $_GET['accesscheck'];
}

if (isset($_POST['usuario'])) {
 $loginUsername=$_POST['usuario'];
 $password=md5($_POST['senha']);
 $MM_fldUserAuthorization = "usuarioNivel";
 $MM_redirectLoginSuccess = "admin/painel.php";
 $MM_redirectLoginFailed = "admin/erro.php";
 $MM_redirecttoReferrer = false;
 mysql_select_db($database_painel_config, $painel_config);

 $LoginRS__query=sprintf("SELECT emailCliente, senhaCliente, usuarioNivel FROM ms_clientes WHERE emailCliente=%s AND senhaCliente=%s",
 GetSQLValueString($loginUsername, "text"), GetSQLValueString($password, "text"));

 $LoginRS = mysql_query($LoginRS__query, $painel_config) or die(mysql_error());
 $loginFoundUser = mysql_num_rows($LoginRS);
 if ($loginFoundUser) {

 $loginStrGroup = mysql_result($LoginRS,0,'usuarioNivel');

if (PHP_VERSION >= 5.1) {session_regenerate_id(true);} else {session_regenerate_id();}
//declare two session variables and assign them

 $_SESSION['MM_Username'] = $loginUsername;
 $_SESSION['MM_UserGroup'] = $loginStrGroup;

 if (isset($_SESSION['PrevUrl']) && false) {
 $MM_redirectLoginSuccess = $_SESSION['PrevUrl'];
 }
 header("Location: " . $MM_redirectLoginSuccess );
 }
 else {
 header("Location: ". $MM_redirectLoginFailed );
 }
}
?>

O erro é:Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at /home/portalno/public_html/header.php:1) in /home/portalno/public_html/header.php on line 36

A linha 36 é onde verifico se existe uma sessão...

if (!isset($_SESSION)) {
 session_start(); -> linha 36
}