Overnaturligt om "Hjæælp! Warning: session_start()"

Hej alle. Har dette problem her, og jeg ved simpelthen ikke hvad jeg skal gøre mere.
I ser fejlen her: Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /mounted-storage/home131/sub003/sc73620-JKUC/overnaturligt.dk/wp-config.php:1) in /mounted-storage/home131/sub003/sc73620-JKUC/overnaturligt.dk/wp-content/themes/diablofun/functions.php on line 20

Og her er line 1 - 20 i functions.php
<?php

if ( ( $pagenow == "themes.php" ) && current_user_can('administrator') && isset( $_GET['activated'] ) && ( $_GET['activated']=='true' ) ) {
if (!include_once( get_template_directory() . '/inc/activation.php' )) {
$stylesheet = get_option( 'theme_switched' );
switch_theme( $stylesheet, $stylesheet );
if ( file_exists( get_template_directory() . '/inc/activation.php' ) ) {
$error_message[]="PHP hasn't access to file /inc/activation.php. Check the file permissions and try again please.";
} else {
$error_message[]="File /inc/activation.php doesn't exists. Try to upload the theme again.";
}
$message="<h2>The theme wasn't activated by following reasons:</h2>

• ".implode("
• ",$error_message)."

if (!session_id()) {
session_start();}