I spotted the issue, but I still can't figure out why these lines are important:
$aUrl = parse_url( $site['url'] );
if( isset($_SERVER['HTTP_HOST']) and $_SERVER['HTTP_HOST'] != $aUrl['host'] and $_SERVER['HTTP_HOST'] != $aUrl['host'] . ':80' )
{
header( "Location:http://{$aUrl['host']}{$_SERVER['REQUEST_URI']}" );
exit;
}

I just commented it, but I think I should not have done it!