Just tried it with this Code, which works like a charm:
<?php
$startTime = microtime(true);
$kotomi_indexFile = "./";
$kotomi_container = true;
$fileDir = dirname(__FILE__)."/{$kotomi_indexFile}";
require "{$fileDir}/library/Dark/Kotomi/KotomiHeader.php";
XenForo_Session::startPublicSession()...