Random Microsoft Reporting Services failure

I have a web app which prints Reporting Services reports on demand without displaying them (using LocalReports). It has worked for months but started failing the other day with this exception:

System.MissingMethodException: Method not found: 'Void Microsoft.Reporting.PreviewItemContext.SetPath(System.String, System.String, Microsoft.Reporting.DefinitionSource, System.Reflection.Assembly)'. at Microsoft.Reporting.WebForms.LocalReport.CreateItemContext() at Microsoft.Reporting.WebForms.LocalReport.CompileReport() at Microsoft.Reporting.WebForms.LocalReport.InternalRender(String format, Boolean allowInternalRenderers, String deviceInfo, CreateAndRegisterStream createStreamCallback, Warning[]& warnings) at Microsoft.Reporting.WebForms.LocalReport.Render(String format, String deviceInfo, CreateStreamCallback createStream, Warning[]& warnings) at PipelineWeb.ReportManager.Export(LocalReport report) at PipelineWeb.DispatchService.PrintShippingReport(String SalesOrderNumber, DateTime TransactionDate)

It still works on my development machine. I'm stumped so far.

Update: Thanks to a helpful MS employee on the forums, this issue has been resolved. It had to do with conflicting ReportViewer dlls following the VS SP1 update. Details here -http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=1131480&SiteID=1