An error occured while sending the request (Inner Exception: The underlying conection was closed: An unexpected error occured on a send)

public async Task<string> GetToken(string username, string password)
    {
        using (var client = new HttpClient())
        {
            InitializeHttpClient(client);

            HttpContent requestContent = new StringContent("grant_type=password&username=" + username + "&password=" + password, Encoding.UTF8, "application/x-www-form-urlencoded");

            var response = await client.PostAsync("token", requestContent);

            if (response == null || response.StatusCode == HttpStatusCode.BadRequest)
                return null;

            if (response.StatusCode == HttpStatusCode.NotFound
                || response.StatusCode == HttpStatusCode.RequestTimeout
                || response.StatusCode == HttpStatusCode.BadGateway
                || response.StatusCode == HttpStatusCode.ServiceUnavailable)
            {
                throw new Exception("No Connection to Web Service");
            }

            var bearerData = response.Content.ReadAsStringAsync().Result;

            return JObject.Parse(bearerData)["access_token"].ToString();
        }
    }


     private void InitializeHttpClient(HttpClient client)
    {
        client.BaseAddress = new Uri(_webServiceAddress);
        client.DefaultRequestHeaders.Accept.Clear();
        client.DefaultRequestHeaders.Add("User-Agent", "Test Client");
    }

    private void InitializeHttpClient(HttpClient client, string token)
    {
        InitializeHttpClient(client);
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("bearer", token);

    }