XML-RPC, Error 403 Forbidden

[eluser]enigmas[/eluser]
Hello, anybody faced the problem when the server part, written as a XML-RPC service with CodeIgniter returned "Error 403 Forbidden"?

I'm trying to access to the server XML-RPC service with my C# application, but always get this error.

But other pages on the site are working well. Moreover, XML-RPC server + C# application are working well on localhost and on the one of the servers, but on the second server I get this error.. What can be a reason?