Why Yammer is giving "(500) Internal Server Error." ?

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C#

.NET

REST

Any idea why while posting messages, Yammer is throwing "The remote server returned an error: (500) Internal Server Error." error. and the message is not posted. Any help..?

I'm using Yammer .NET wrapper and it's working fine with REST call "https://www.yammer.com/api/v1/messages.json", but posting message gives the above error. Any workarounds or experience on this?, thanks.

 String consumerkey = "consumerkey";
        String clientsecret = "clientsecret";
        WebProxy webpro = new WebProxy("ourproxyserver",8080)
        webpro.BypassProxyOnLocal = true;
        webpro.Credentials = CredentialCache.DefaultCredentials;
        String access_token = "https://www.yammer.com/oauth2/access_token.json"+
            "?client_id=clientid&"+
            "client_secret=cli_secret&code=code_id";

        Yammer.Session session = new Yammer.Session(new OAuth.OAuthKey(consumerkey, clientsecret,
            access_token,"token")
            , webpro);

            NameValueCollection parameters = new NameValueCollection();
            parameters.Add("body", body);

response = Yammer.HttpUtility.Post(Resources.YAMMER_MESSAGES_CREATE, parameters, session);