Problemas com session_start

Olá galera eu estou com um problema de session, o site no meu servidor local funcionou normal mas quando passei para a minha hospedagem dá o seguinte erro:

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/kaioa758/public_html/afabricadaweb/admin/painel.php:1) in /home/kaioa758/public_html/afabricadaweb/admin/scripts/restrict_no.php on line 4

A linha do erro é essa: session_start();

A página com o código é essa:

<?php
//initialize the session

if (!isset($_SESSION)) {
 session_start();
}

// Logout the current user.

$logoutAction = $_SERVER['PHP_SELF']."?doLogout=true";
if ((isset($_SERVER['QUERY_STRING'])) && ($_SERVER['QUERY_STRING'] != "")){
 $logoutAction .="&". htmlentities($_SERVER['QUERY_STRING']);
}

if ((isset($_GET['doLogout'])) &&($_GET['doLogout']=="true")){
 //to fully log out a visitor we need to clear the session varialbles
 $_SESSION['MM_Username'] = NULL;
 $_SESSION['MM_UserGroup'] = NULL;
 $_SESSION['PrevUrl'] = NULL;
 unset($_SESSION['MM_Username']);
 unset($_SESSION['MM_UserGroup']);
 unset($_SESSION['PrevUrl']);

 $logoutGoTo = "index.php";
 if ($logoutGoTo) {
 header("Location: $logoutGoTo");
 exit;
 }
}
?>
<?php
if (!isset($_SESSION)) {
 session_start();
}
$MM_authorizedUsers = "";
$MM_donotCheckaccess = "true";

// *** Restrict Access To Page: Grant or deny access to this page

function isAuthorized($strUsers, $strGroups, $UserName, $UserGroup) {
 // For security, start by assuming the visitor is NOT authorized.
 $isValid = False;

 // When a visitor has logged into this site, the Session variable MM_Username set equal to their username.
 // Therefore, we know that a user is NOT logged in if that Session variable is blank.
 if (!empty($UserName)) {
 // Besides being logged in, you may restrict access to only certain users based on an ID established when they login.
 // Parse the strings into arrays.
 $arrUsers = Explode(",", $strUsers);
 $arrGroups = Explode(",", $strGroups);
 if (in_array($UserName, $arrUsers)) {
 $isValid = true;
 }
 // Or, you may restrict access to only certain users based on their username.
 if (in_array($UserGroup, $arrGroups)) {
 $isValid = true;
 }
 if (($strUsers == "") && true) {
 $isValid = true;
 }
 }
 return $isValid;
}

$MM_restrictGoTo = "index.php";
if (!((isset($_SESSION['MM_Username'])) && (isAuthorized("",$MM_authorizedUsers, $_SESSION['MM_Username'], $_SESSION['MM_UserGroup'])))) {
 $MM_qsChar = "?";
 $MM_referrer = $_SERVER['PHP_SELF'];
 if (strpos($MM_restrictGoTo, "?")) $MM_qsChar = "&";
 if (isset($_SERVER['QUERY_STRING']) && strlen($_SERVER['QUERY_STRING']) > 0)
 $MM_referrer .= "?" . $_SERVER['QUERY_STRING'];
 $MM_restrictGoTo = $MM_restrictGoTo. $MM_qsChar . "accesscheck=" . urlencode($MM_referrer);
 header("Location: ". $MM_restrictGoTo);
 exit;
}
?>