Twitter/search API 500 Internal error


I use twitter api and mvc 3 to search tweets by hashtag, I use this code

string codes = "XXXXXXXX";
        byte[] arrCodes = System.Text.Encoding.Unicode.GetBytes(codes);
        string base64Codes = Convert.ToBase64String(arrCodes);
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3;

            var webRequest = (HttpWebRequest)WebRequest.Create(@"");
            webRequest.Method = "Post";
     //    webRequest.Host = "";
         webRequest.UserAgent = "NewProjapi";
            webRequest.Headers.Add("Authorization: Basic "+base64Codes);
            webRequest.Headers.Add("Accept-Encoding: gzip");
            string postString = string.Format("grant_type=client_credentials");
            byte[] postStringBytes = System.Text.Encoding.ASCII.GetBytes(postString);

            webRequest.ContentType = "application/x-www-form-urlencoded;charset=UTF-8";
            webRequest.ContentLength = postStringBytes.Length;
            Stream requestStream = webRequest.GetRequestStream();
        requestStream.Write(postStringBytes, 0, postStringBytes.Length);
        var webResponse = webRequest.GetResponse();
        byte[] res = new byte[webResponse.ContentLength];
        webResponse.GetResponseStream().Read(res, 0, res.Length);

but in line var webResponse = webRequest.GetResponse();, it said that The remote server returned an error: (500) Internal Server Error. I change my code and use Webclient but it still say that. Twitter support tells we resolve this problem but I have it still.
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