The Report Viewer Web Control HTTP Handler has not been registered in the application's web.config file. Add <add verb="*" path="Reserved.ReportViewerWebPart.axd" type = "Microsoft.ReportingServices.SharePoint.UI.WebParts.WebPartHttpHandler, Microsoft.ReportingServices.SharePoint.UI.WebParts, Version=12.0.0.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91" /> to the system.web/httpHandlers section of the web.config file, or add <add name="ReportViewerWebControlHandler" preCondition="integratedMode" verb="*" path="Reserved.ReportViewerWebPart.axd" type="Microsoft.ReportingServices.SharePoint.UI.WebParts.WebPartHttpHandler, Microsoft.ReportingServices.SharePoint.UI.WebParts, Version=12.0.0.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91" /> to the system.webServer/handlers section for Internet Information Services 7 or later.