An error occurred during local report processing. The definition of the report 'Main Report' is invalid. The report definition is not valid. Details: The report definition has an invalid target namespace 'http://schemas.microsoft.com/sqlserver/reporting/2008/01/reportdefinition' which cannot be upgraded.

When I was running a Report I was getting the following Error.

An error occurred during local report processing. 
The definition of the report 'Main Report' is invalid. 
The report definition is not valid. Details: The report definition has an invalid target namespace 'http://schemas.microsoft.com/sqlserver/reporting/2008/01/reportdefinition' which cannot be upgraded. 

Actually the report was developed in VS 2008 and now, I am trying to access the report in VS2010 then I was getting this Error.

 To solve this,

First go to web.config and modified the following lines.

Before modifying add the following reference.

As follows.

And click on Ok button.

<assemblies>

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

</assemblies>

<httpHandlers>

<add verb="*"

 path="Reserved.ReportViewerWebControl.axd" type =

 "Microsoft.Reporting.WebForms.HttpHandler,

 Microsoft.ReportViewer.WebForms, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />

</httpHandlers>

  <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>

And go to the Report page and modified the following lines of code.

<%--<%@ Register Assembly="Microsoft.ReportViewer.WebForms, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" Namespace="Microsoft.Reporting.WebForms" TagPrefix="rsweb" %>--%>

<%@ RegisterAssembly="Microsoft.ReportViewer.WebForms, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"

    Namespace="Microsoft.Reporting.WebForms"TagPrefix="rsweb"%>