Ошибка регистрации в instagram через прокси

   public static CookieContainer auth(string login, string pass,string email,string first_name,WebProxy prox)
        {
            T:
            Random rnd = new Random();
            login += rnd.Next(1232);
            email = login + "@mail.ru";
            string USER_AGENT = "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:49.0) Gecko/20100101 Firefox/49.0";
            var client = new RestClient("https://www.instagram.com/");
 
            client.Proxy = prox;
            client.CookieContainer = new CookieContainer();
            client.UserAgent = USER_AGENT;
 
 
            var firstRequest = new RestRequest("/", Method.POST);
            var firstResponse = client.Execute(firstRequest);
 
            var csrftoken = firstResponse.Cookies.First(x => x.Name == "csrftoken").Value;
            var loginRequest = new RestRequest("/accounts/web_create_ajax/", Method.POST);
            loginRequest.AddHeader("X-CSRFToken", csrftoken);
            loginRequest.AddHeader("X-Requested-With", "XMLHttpRequest");
            loginRequest.AddHeader("X-Instagram-AJAX", "1");
            loginRequest.AddHeader("Referer", client.BaseUrl.ToString());
            loginRequest.AddHeader("Accept", "*/*");
            loginRequest.AddHeader("Accept-Language", "ru-RU,ru;q=0.8,en-US;q=0.6,en;q=0.4");
            loginRequest.AddHeader("Accept-Encoding", "gzip, deflate, br");
            loginRequest.AddHeader("Content-Type", "application/json");
            loginRequest.AddHeader("Origin", "https://www.instagram.com");
            loginRequest.AddHeader("Host", "www.instagram.com");
            loginRequest.AddParameter("email", email);
            loginRequest.AddParameter("password", pass);
            loginRequest.AddParameter("username", login);
            loginRequest.AddParameter("first_name", first_name);
            var loginResponse = client.Execute(loginRequest);
            string cooks = loginResponse.Content;
            if (cooks.Contains("that username is taken") == true || cooks.Contains("errors":") == true)
            {
                login += rnd.Next(1232);
                email = login + "@mail.ru";
                goto T;
            }
            File.AppendAllText("result.txt", login + ":" + email + ":" + pass + ":" + first_name);
                return client.CookieContainer;
 
 
        }