How can I debug this web service?

I have a simple web service that looks something like this:

[WebMethod]
public OrderForecastItem GetOrderForecast(int shipTo, string catalogName, bool showPricing)
{ 
    return OrderForecastManager.GetOrderForecast(shipTo, catalogName, showPricing);
}

I call it from another place in a fairly simple way:

using (OrderForecastWS.OrderForecastWS service = new OurSite.Web.Reporting.OrderForecastWS.OrderForecastWS())
{
    OrderForecastItem orderForecastItems = service.GetOrderForecast(2585432, "DENTAL", false);
}

After some gymnastics, so that the systems understand that I'm talking about the same type of objects on the client and server side (I had to open the file Reference.csinside mine Web References, delete the generated one OrderForecastItemand add a link to our real one OrderForecastItem), the system starts up and tries to get the element.

Unfortunately, now he is bombing during a call to the service, stating:

Exception There is an error in the XML document (1, 1113). (InvalidOperationException)

- , , , , XML. ( , XML W3Schools, .)

, ... , , XML-, , (1, 1113) .

, , , , .

?