ReportViewer - strange not shown exception when creating the report

Hi,

I'm using the WebForms version of the ReportViewer in LocalMode. In the structure I have one main report with 5 subreports. Generation of the report is really slow (1800 report pages for around 10 minutes). While debugging, when I stop and I see the executing thread, the location is most of the time in

Microsoft.ReportingServices.DataExtensions.MappingDataReader.GenerateFieldErrorException()

I can't find anywhere in the web what exactly this method do, and most especially, what is the cause for this FieldErrorException. By the way, no exception is thrown during the whole process of creating the report and that's the only info I have.