Friday, November 29, 2019

ASP.NET ReportViewer - ReportViewerWebControl.axd was not found

After migrating an old ASP.NET application to a new IIS server, the application started to throw errors. The app was using a ReportViewer control to show some SQL Server Reporting Services reports. But when I tried to load a specific report, the report viewer kept showing a spinning wheel.

When I took a look at the console, I noticed 404 errors for ReportViewerWebControl.axd.

The problem was that the original application was hosted on IIS 6. To configure the ReportViewer in IIS6 an entry for the Http Handler was added inside the system.web <httpHandler> section. Starting from IIS 7.0 this section is no longer read. Instead we have to add the ReportViewerWebControl.axd to the system.webserver <handlers> section: