Silverlight WCF Web Service call returns "The remote server returned an error: NotFound"

Hi ,

I have a webservice with 2 methods - one method that takes one paramter and returns an ObervableCollection<>, and another that takes the same ObervableCollection<> with modified values to be submitted to the database, and returns bool.

The 1st call to retrieve the data works fine, the data is bound to a DataGrid, values are changed no problem etc... When I call the 2nd method and pass the ObervableCollection<> back, the web service throws a CommunicationException. Any ideas why this would happen?

Stack Trace:

System.ServiceModel.CommunicationException was unhandled by user code
  Message="The remote server returned an error: NotFound"
  StackTrace:
       at System.ServiceModel.AsyncResult.End[TAsyncResult](IAsyncResult result)
       at System.ServiceModel.Channels.ServiceChannel.SendAsyncResult.End(SendAsyncResult result)
       at System.ServiceModel.Channels.ServiceChannel.EndCall(String action, Object[] outs, IAsyncResult result)
       at System.ServiceModel.ClientBase`1.ChannelBase`1.EndInvoke(String methodName, Object[] args, IAsyncResult result)
       at MarketBuilder.WebService.WebServiceClient.WebServiceClientChannel.EndCommitChanges(IAsyncResult result)
       at MarketBuilder.WebService.WebServiceClient.MarketBuilder.WebService.WebService.EndCommitChanges(IAsyncResult result)
       at MarketBuilder.WebService.WebServiceClient.OnEndCommitChanges(IAsyncResult result)
       at System.ServiceModel.ClientBase`1.OnAsyncCallCompleted(IAsyncResult result)
  InnerException: System.Net.WebException
       Message="The remote server returned an error: NotFound"
       StackTrace:
            at System.Net.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state)
            at System.Net.BrowserHttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
            at System.ServiceModel.Channels.HttpChannelFactory.HttpRequestChannel.HttpChannelAsyncRequest.CompleteGetResponse(IAsyncResult result)
       InnerException: System.Net.WebException
            Message="The remote server returned an error: NotFound"
            StackTrace:
                 at System.Net.BrowserHttpWebRequest.InternalEndGetResponse(IAsyncResult asyncResult)
                 at System.Net.BrowserHttpWebRequest.<>c__DisplayClass5.<EndGetResponse>b__4(Object sendState)
                 at System.Net.AsyncHelper.<>c__DisplayClass2.<BeginOnUI>b__0(Object sendState)
            InnerException: