There is a difference between IIS and Apache concerning a line in includes\session.php

see also
http://www.phpbb.com/community/viewtopic.php?f=72&t=1555195&p=9405715#p9405715

This is the fix:

if ($config['board_disable'] && !defined('IN_LOGIN') && !$auth->acl_gets('a_', 'm_') && !$auth->acl_getf_global('m_'))
{
// this bug must be commented out: header('HTTP/1.1 503 Service Unavailable');

$message = (!empty($config['board_disable_msg'])) ? $config['board_disable_msg'] : 'BOARD_DISABLE';
trigger_error($message);
}