Sharepoint 2013 Online Remote Event Receiver Error

Hello. I write remote event receiver for SharePoint 2013 online portal. I get this error in class TokenHelper:  "Unable to connect to remote server" Could anybody explain what is the problem, please? The error fires in this block of code : 

  acsMetadata = webClient.DownloadData(acsMetadataEndpointUrlWithRealm);
 private static JsonMetadataDocument GetMetadataDocument(string realm)
            {
                string acsMetadataEndpointUrlWithRealm = String.Format(CultureInfo.InvariantCulture, "{0}?realm={1}",
                                                                        GetAcsMetadataEndpointUrl(),
                                                                        realm);
                byte[] acsMetadata;
                using (WebClient webClient = new WebClient())
                {

                    acsMetadata = webClient.DownloadData(acsMetadataEndpointUrlWithRealm);
                }
                string jsonResponseString = Encoding.UTF8.GetString(acsMetadata);

                JavaScriptSerializer serializer = new JavaScriptSerializer();
                JsonMetadataDocument document = serializer.Deserialize<JsonMetadataDocument>(jsonResponseString);

                if (null == document)
                {
                    throw new Exception("No metadata document found at the global endpoint " + acsMetadataEndpointUrlWithRealm);
                }

                return document;
            }

 try
            {
                oauth2Response =
                    client.Issue(AcsMetadataParser.GetStsUrl(targetRealm), oauth2Request) as OAuth2AccessTokenResponse;
            }
            catch (WebException wex)
            {
                using (StreamReader sr = new StreamReader(wex.Response.GetResponseStream()))
                {
                    string responseText = sr.ReadToEnd();
                    throw new WebException(wex.Message + " - " + responseText, wex);
                }
            }