Echec d'un GetRequestStream sur requête HTTP post

Bonjour,

J'ai un service windows qui envoie des requête HTTPS vers un terminal.

Si je donne au service les droits administrateur au service via services.msc tout va bien.
Si je donne les droits LocalSystem, j'ai une erreur 403 Forbidden sur un GetRequestStream

Citation:

ERROR STACK TRACE ===>System.Net.WebException: The remote server returned an error: (403) Forbidden.
at System.Net.HttpWebRequest.GetRequestStream(TransportContext& context)
at System.Net.HttpWebRequest.GetRequestStream()

Merci d'avance.

Code:

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		 String credentials = System.Convert.ToBase64String(System.Text.Encoding.GetEncoding("ISO-8859-1").GetBytes(user + ":" + pwd));
                 ServicePointManager.Expect100Continue = true;
                 ServicePointManager.SecurityProtocol =  SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls;
 
 
                string url;
                if (pQueryType == QueryType.post)
                {
                    url = string.Format("https://{0}//putxml", ipAddress);
                }
                else
                {
                    url = string.Format("https://{0}/getxml?{1}", ipAddress, pQuery);
                }
 
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
                request.Headers.Add("Authorization", "Basic " + credentials);
                request.UserAgent=  "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:25.0) Gecko/20100101 Firefox/25.0";
                request.Method = pQueryType == QueryType.post ? "POST" : "GET"; 
 
 
                byte[] bytes;
 
                if (pQueryType == QueryType.post)
                { 
 
                    bytes = System.Text.Encoding.UTF8.GetBytes(pQuery);
                    request.ContentType = "application/x-www-form-urlencoded";
                    request.ContentLength = bytes.Length;
                    request.Timeout = 3000;
                    request.ProtocolVersion = HttpVersion.Version11;
                    request.KeepAlive = false; 
                    request.Accept = "*/*";
 
                    if (request.Proxy != null)
                    {
                        request.Proxy.Credentials = CredentialCache.DefaultCredentials;
                    }
 
                    request.AllowWriteStreamBuffering = false;
                    request.Credentials = CredentialCache.DefaultCredentials;
 
                    Stream requestStream = request.GetRequestStream();  // <<<ECHEC ICI
                    requestStream.Write(bytes, 0, bytes.Length);
                    requestStream.Close();
                }
 
                HttpWebResponse response = request.GetResponse() as HttpWebResponse;
 
                if (response.StatusCode == HttpStatusCode.OK)
                {
                    Stream responseStream = response.GetResponseStream();
                    string responseStr = new StreamReader(responseStream).ReadToEnd();
                    result = true;
 
                    pResponse = responseStr;
                    responseStream.Close();
                }