(401) Unauthorized Error working with Client Object Model

Question

Hi All, I'm tryiing to retrieve a list of document libraries from a sharepoint site, here is my code, this is a windows app

public string[] GetDocumentLibraries(ClientContext ctx) { Collection libraries = new Collection(); try { //Getting the site from the SP context Web oWebsite = ctx.Web; write("INFO: SP site gotten");

            //Getting the list collection from the SP site
            write("INFO: Getting the list collection from the SP site");
            ListCollection listCollect = oWebsite.Lists;
            write("INFO: Loading the list");
            ctx.Load(listCollect);
            write("INFO: Getting the list");
            ctx.ExecuteQuery();
            write("INFO: List Collection size: " + listCollect.Count);

            //Getting the list of document libraries
            foreach (List docLibList in oWebsite.Lists)
            {
                if (docLibList.BaseTemplate == (int)ListTemplateType.DocumentLibrary)
                {
                    write("INFO: Document Library: " + docLibList.Title);
                    libraries.Add(docLibList.Title);
                }
            }
        }
        catch (Exception e)
        {
            write("ERROR: Error getting the list of document libraries, error detail " + e.Message + " " + e.StackTrace);
        }

        return libraries.ToArray();
    }

I had tried this code in three different SharePoint servers, it works in two of them, but in the third one I'm getting this exception

ERROR: Error getting the list of document libraries, error detail The remote server returned an error: (401) Unauthorized. at System.Net.HttpWebRequest.GetResponse() at Microsoft.SharePoint.Client.SPWebRequestExecutor.Execute() at Microsoft.SharePoint.Client.ClientContext.EnsureFormDigest() at Microsoft.SharePoint.Client.ClientContext.ExecuteQuery() at P_DocumentImporter.DocumentImporter.GetDocumentLibraries(ClientContext ctx)

And I'm sure the credentials are correct,

Any clue about this,

Thanks,