Taxonomy.asmx returns sometimes 'No credentials are available in the security package'

Question

I've created a WCF WebService which connects to the Taxonomy service from SharePoint with a 'Service Reference' in Visual Studio 2010 to the https://localhost/sites/MYSITE/_vti_bin/TaxonomyClientService.asmx service.

But when calling the method GetTermSets(...) I do sometimes get this exception:

System.ServiceModel.ProtocolException: The content type text/html; charset=utf-8 of the response message does not match the content type of the binding (text/xml; charset=utf-8). If using a custom encoder, be sure that the IsContentTypeSupported method is implemented properly. The first 52 bytes of the response were: 'No credentials are available in the security package'.

Server stack trace: 
   at System.ServiceModel.Channels.HttpChannelUtilities.ValidateRequestReplyResponse(HttpWebRequest request, HttpWebResponse response, HttpChannelFactory factory, WebException responseException, ChannelBinding channelBinding)
   at System.ServiceModel.Channels.HttpChannelFactory.HttpRequestChannel.HttpChannelRequest.WaitForReply(TimeSpan timeout)
   at System.ServiceModel.Channels.RequestChannel.Request(Message message, TimeSpan timeout)
   at System.ServiceModel.Dispatcher.RequestChannelBinder.Request(Message message, TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs, TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(IMethodCallMessage methodCall, ProxyOperationRuntime operation)
   at System.ServiceModel.Channels.ServiceChannelProxy.Invoke(IMessage message)

Any idea why this happens ?

Answer 1

It seems that when I switched to the old 2.0 .NET compatible 'Web Reference', the problem was solved.

public partial class Taxonomywebservice
{
  public Taxonomywebservice(BusinessConfig config) : base()
  {
    base.Url = config.Taxonomy;
    base.Credentials = new NetworkCredential(config.UserName, config.Password, config.Domain);
    base.PreAuthenticate = true;
  }
}

I don't have a good explanation for this, maybe the PreAuthenticate = true solves the issue ?