The Error:
The Report Viewer Web Control HTTP Handler has not been registered in the application's web.config file. Add to the system.web/httpHandlers section of the web.config file, or add to the system.webServer/handlers section for Internet Information Services 7 or later.
The Web config file :
<system.web>
<authentication mode="None" />
<compilation debug="true" targetFramework="4.5.2" />
<httpRuntime targetFramework="4.5.2" />
<httpHandlers>
<add path="Reserved.ReportViewerWebControl.axd" verb="*" type="Microsoft.Reporting.WebForms.HttpHandler, Microsoft.ReportViewer.WebForms, Version=11.0.0.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91" validate="false" />
</httpHandlers>
</system.web>
<system.webServer>
<modules>
<remove name="FormsAuthentication" />
</modules>
<validation validateIntegratedModeConfiguration="false" />
<handlers>
<remove name="ReportViewerWebControlHandler"/>
<add name="ReportViewerWebControlHandler" preCondition="integratedMode" verb="*" path="Reserved.ReportViewerWebControl.axd" type="Microsoft.Reporting.WebForms.HttpHandler, Microsoft.ReportViewer.WebForms, Version=11.0.0.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91" />
</handlers>
</system.webServer>
The config entries do exists, but the error is still thrown.