HTTP/1.1 401 Не авторизован при использовании клиента и сервера PHP XML-RPC

Question

HTTP/1.1 401 Unauthorized

This server could not verify that you are authorized to access the document
you requested.  Either you supplied the wrong credentials (e.g., bad 
password), or your browser does not understand how to supply the 
credentials required. 

Fault Code: 5, Reason: Didn't receive 200 OK from remote server. 
(HTTP/1.1 401 Unauthorized) 

Я пытаюсь получить доступ к серверу API через его IP-адрес по протоколу PHP XML-RPC. Почему сервер выдает эту ошибку?