This is now fixed. For some reason the PHP dirname function was behaving inconsistently on your server. I have modified the code so that it no longer uses that function. Specifically, what I changed was in the file /administrator/components/com_nbill/framework/interop/nbill.interop.abstract.php, around line 357 where it said:
$live_site .= str_replace("\\", "/", dirname($url_parts['path']));
...I changed it to:
$live_site .= str_replace("\\", "/", $url_parts['path']);
//Knock off trailing slash and anything that comes after it
$live_site = substr($live_site, 0, strrpos($live_site, "/"));
This fix will be included in the next release.