Login.live.com Server not found Error

Question

I'm using Microsoft Graph API to authenticate users to my web application. For SO, I have registered a test application in Registration Portal. This portal provide us application id and application secret. I have defined redirect URL which take Code and return the access token.

 public async Task<AuthToken> ExchangeCodeForAccessToken(string code)
    {
        MicrosoftGraphTokenResponse tokenResponse = null;
        using (HttpClient client = new HttpClient())
        {
            var postFormParameters = new List<KeyValuePair<string, string>>()
            {
                    new KeyValuePair<string, string>("client_id", _msGraphpParameter.AppId),
                    new KeyValuePair<string, string>("redirect_uri", _msGraphpParameter.RedirectUrl),
                    new KeyValuePair<string, string>("code", code),
                    new KeyValuePair<string, string>("grant_type", "authorization_code"),
                    new KeyValuePair<string, string>("scope", _msGraphpParameter.Scopes)
            };

            if (_msGraphpParameter.AppSecret != null)
            {
                // Secret isn't needed for client apps
                postFormParameters.Add(new KeyValuePair<string, string>("client_secret", _msGraphpParameter.AppSecret));
            }

            var formContent = new FormUrlEncodedContent(postFormParameters);
            client.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", TokenRefreshContentType);

            HttpResponseMessage response = await client.PostAsync(MsGraphTokenRefreshUrl, formContent);
            string responseString = await response.Content.ReadAsStringAsync();
            tokenResponse = JsonConvert.DeserializeObject<MicrosoftGraphTokenResponse>(responseString);

        }
        return TransformMSGraphTokenResponseIntoAuthProperties(tokenResponse);
    }

My problem is bit weird. I'm able to access the token without any issues which means I have correct setup in place. However, after few calls(2) Graph API return nothing as login live dot com server not found.

I have tried cleaning cookies completely, dns cache but it didn't worked however if I restart my laptop and place a call then it seems start working. Still after few calls, Login.live.com server not found.

Here is snapshot

Please let me know your thoughts. Thanks