private void button2_Click(object sender, EventArgs e)
    {
        string formUrl = "https://declaraciones.sri.gob.ec/tuportal-internet/j_security_check";
        string formParams = String.Format("j_username={0}&j_cedula=&j_password={1}&loguear=si&boton=Aceptar", "USER", "PASSWORD");

        string cookieHeader;

        var cookies = new CookieContainer();
        HttpWebRequest req = (HttpWebRequest)WebRequest.Create(formUrl);
        req.CookieContainer = cookies;

        req.ContentType = "application/x-www-form-urlencoded";
        req.Method = "POST";
        byte[] bytes = Encoding.ASCII.GetBytes(formParams);
        req.ContentLength = bytes.Length;
        using (Stream os = req.GetRequestStream())
        {
            os.Write(bytes, 0, bytes.Length);
        }
        WebResponse resp = req.GetResponse();   // AQUI EL ERROR

        cookieHeader = resp.Headers["Set-cookie"];

        string pageSource1;
        string pageSource2;
        string getUrl = "https://declaraciones.sri.gob.ec/comprobantes-electronicos-internet/pages/consultas/recibidos/comprobantesRecibidos.jsf";

        HttpWebRequest getRequest = (HttpWebRequest)WebRequest.Create(getUrl);
        getRequest.CookieContainer = new CookieContainer();

        getRequest.Headers.Add("Cookie", cookieHeader);
        WebResponse getResponse = getRequest.GetResponse();
        using (StreamReader sr = new StreamReader(getResponse.GetResponseStream()))
        {
            pageSource1 = sr.ReadToEnd();
        }
        using (StreamWriter outfile =
       new StreamWriter("Button2Page1.html"))
        {
            outfile.Write(pageSource1.ToString());
        }

        using (StreamReader sr = new StreamReader(resp.GetResponseStream()))
        {
            pageSource2 = sr.ReadToEnd();
        }

        using (StreamWriter outfile =
     new StreamWriter("Button2Page2.html"))
        {
            outfile.Write(pageSource2.ToString());
        }
    }