SL2B2: ADO.Net Data Services fails to BeginnSaveChanges

 Hi, im trying to sort out some issues with Ado.net data services.
 
The following code throws me an exception:
 try
            {
                product _product = new product();
                _product.product_article_number = "xy1234";
                sachariaEntities _sacharia = new sachariaEntities(new Uri(_serviceUriString, UriKind.RelativeOrAbsolute));
                _sacharia.MergeOption = MergeOption.AppendOnly;
                _sacharia.AddObject("product", _product) ...