Ref:
Add the following to <system.web> => <httpHandlers> section.
<add verb = "*" path = "Reserved.ReportViewerWebControl.axd" type = "Microsoft.Reporting.WebForms.HttpHandler, Microsoft.ReportViewer.WebForms, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
Add the following to <system.webServer> => <handlers> section.
<add name = "ReportViewerWebControlHandler" preCondition = "integratedMode" verb = "*" path = "Reserved.ReportViewerWebControl.axd" type = "Microsoft.Reporting.WebForms.HttpHandler, Microsoft.ReportViewer.WebForms, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />