Oauth/token server 400 error

Hi,

I have to many errors when I look at analytics tab - endpoints. Most of them are oauth/token post,

Application type is Server-Only.

I am using c# client and code is below.

rc = new RestClient(AppKey, AppSecretKey, Server);

await rc.Authorize(Username, Extension, Password);

var account = rc.Restapi().Account();

var extension = account.Extension();

var response = await extension.MessageStore().List();

The error I've got is:

A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond 192.209.29.78:443

Flurl.Http.FlurlHttpException: Call failed. An error occurred while sending the request. GET https://platform.devtest.ringcentral.com/restapi/v1.0/account/~/extension/~/message-store ---> System.Net.Http.HttpRequestException: An error occurred while sending the request. ---> System.Net.WebException: Unable to connect to the remote server ---> System.Net.Sockets.SocketException: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond 192.209.29.78:443