I got this error using ado.net data services “An error occurred while processing this request.”

Question

Any idea?

protected void Button1_Click(object sender, EventArgs e)
{
  PortalCoporativoDataContext msdc = new PortalCoporativoDataContext(new Uri("http://win-j8t24f4b519/MySite/_vti_bin/listdata.svc"));

  var excitingAnnouncements = from announcement in msdc.Announcements
                                        //where announcement.Title.EndsWith("!")
                                        select announcement;

  List<AnnouncementsItem> allAnnouncements = msdc.Announcements.ToList();

  foreach (var announcement in excitingAnnouncements)
  {
    announcement.Title += "!!";
    msdc.UpdateObject(announcement);
  }

  msdc.SaveChanges();
}

I got this on the Event Log:

Event code: 3005 Event message: An unhandled exception has occurred. Event time: 10/4/2010 9:52:36 PM Event time (UTC): 10/5/2010 2:52:36 AM Event ID: 581ab0098f024b7db0354e2754aa0cf7 Event sequence: 8 Event occurrence: 1 Event detail code: 0 Application information: Application domain: /LM/W3SVC/1828559168/ROOT-2-129307206403076171

Trust level: WSS_Minimal 
Application Virtual Path: / 
Application Path: K:\inetpub\wwwroot\wss\VirtualDirectories\803cbd1e1b-10ff-497d-80f1-c477f5867a9c\

Machine name: WIN-J8T24F4B519    Process information: 
Process ID: 1792 
Process name: w3wp.exe 
Account name: WIN-J8T24F4B519\Administrator   

Exception information: Exception type: DataServiceQueryException Exception message: An error occurred while processing this request. Request information: Request URL: http://win-j8t24f4b519/SitePages/Home.aspx

Request path: /SitePages/Home.aspx 
User host address: fe80::81c1:afd3:308:be9f/ 
User: WIN-J8T24F4B519\Administrator 
Is authenticated: True 
Authentication Type: Negotiate 
Thread account name: WIN-J8T24F4B519\Administrator   

Thread information: Thread ID: 10 Thread account name: WIN-J8T24F4B519\Administrator Is impersonating: False Stack trace: at System.Data.Services.Client.DataServiceRequest.Execute[TElement](DataServiceContext context, QueryComponents queryComponents) at System.Data.Services.Client.DataServiceQuery1.Execute() at System.Data.Services.Client.DataServiceQuery1.GetEnumerator() at System.Collections.Generic.List1..ctor(IEnumerable1 collection) at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source) at TrabajandoConDatos.UsandoDataServices.UsandoDataServicesUserControl.Button1_Click(Object sender, EventArgs e) at System.Web.UI.WebControls.Button.OnClick(EventArgs e) at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Custom event details: