The Report Viewer Web Control HTTP Handler has not been registered in the application's web.config file

I was having the very same problem. What happened was I put the Report loading routine on Page_Load, and didn't wrap it in if (!IsPostBack). The ReportViewer makes a POST to the page, and that was triggering Page_Load and reloading the report, somehow messing it up. After putting everything inside if (!IsPostBack), it worked like a charm.

Could you check your web.config for handlers registered or not for ReportViewer. its should be like this

Handler

<handlers>
<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" />
</handlers>

Also check assembly section in your web.config for ReportViewer, it should be like below.

<assemblies>
<add assembly="Microsoft.ReportViewer.WebForms, Version=10.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A" />
   <add assembly="Microsoft.ReportViewer.Common, Version=10.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A" />

The issue also went away for me when I changed the app pool from Integrated to Classic.