Check this article
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Steps
1. Connect to your web site and open the "Handler Mappings" module.
2. From the "Actions" menu, click on the "Add Manager Handler..." option.
3. In the "Add Managed Handler" dialog box that opens up, supply the following values for the fields:
Request Path: Reserved.ReportViewerWebControl.axd
Type: Microsoft.Reporting.WebForms.HttpHandler
Name: ReportViewerWebControl
4. Click on the "OK" button