xbilling - error in frontend

[hackkill CraftHosting]

@[hackkill CraftHosting],

There could be several reasons why your request failed.

Try debugging from inside

Code:

xmwsclient.class.php

What do I debug that part ? 

@[Tgates] Can you help me ?

Code:

function BuildRequest() {
       $request_template = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" .
               "<xmws>" .
               "\t<apikey>" . $this->serverkey . "</apikey>\n" .
               "\t<request>" . $this->method . "</request>\n" .
               "\t<authuser>" . $this->username . "</authuser>\n" .
               "\t<authpass>" . $this->password . "</authpass>\n" .
               "\t<content>" . $this->data . "</content>" .
               "</xmws>";
       return $request_template;
   }