Pessoal o erro esta nessa linha.
byte[] ResponseArray = client.UploadData(URI, "POST", Encoding.UTF8.GetBytes(XMLInstrucao));
erro:
An exception of type 'System.Net.WebException' occurred in System.dll but was not handled in user code
Falha na autenticação porque o participante remoto fechou o fluxo de transporte.
Linha 97: client.Headers.Add("Authorization: Basic " + auth);
Linha 98: client.Headers.Add("User-Agent: Mozilla/4.0");
Linha 99: byte[] ResponseArray = client.UploadData(URI, "POST", Encoding.UTF8.GetBytes(XMLInstrucao));
Linha 100: string sResponse = Encoding.UTF8.GetString(ResponseArray);
Linha 101: #endregion
Marcos Ruiz