Adobe Sign refresh token Issue - getting error The underlying connection was closed

string requestURI = "https://secure.in1.adobesign.com/oauth/v2/refresh";
string refreshToken = refresh token;

var requestBody = new List<KeyValuePair<string, string>>()
{
    new KeyValuePair<string, string>("client_id", Client id),
    new KeyValuePair<string, string>("client_secret", client_secret),
    new KeyValuePair<string, string>("grant_type", "refresh_token"),
    new KeyValuePair<string, string>("refresh_token", refreshToken),
};
using (var httpClient = new HttpClient())
{
    HttpContent requestContent = new FormUrlEncodedContent(requestBody);

    httpClient.DefaultRequestHeaders.Clear();

    var httpResponse = httpClient.PostAsync(requestURI, requestContent).Result;
    var response = httpResponse.Content.ReadAsStringAsync().Result;
}

below is the error details -
System.Net.Http.HttpRequestException: An error occurred while sending the request. ---> System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send. ---> System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host. ---> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host
at System.Net.Sockets.Socket.EndReceive(IAsyncResult asyncResult)
at System.Net.Sockets.NetworkStream.EndRead(IAsyncResult asyncResult)
--- End of inner exception stack trace ---
at System.Net.TlsStream.EndWrite(IAsyncResult asyncResult)
at System.Net.ConnectStream.WriteHeadersCallback(IAsyncResult ar)
--- End of inner exception stack trace ---
at System.Net.HttpWebRequest.EndGetRequestStream(IAsyncResult asyncResult, TransportContext& context)
at System.Net.Http.HttpClientHandler.GetRequestStreamCallback(IAsyncResult ar)
--- End of inner exception stack trace ---

Any pointer will be really helpfull.
Thank you.