Hello,
Do you have the following section in your web.config file?
<system.web>
<pages validateRequest="false" clientIDMode="AutoID">
<controls>
<add tagPrefix="ajaxToolkit" namespace="AjaxControlToolkit" assembly="AjaxControlToolkit" />
What type of application pool are you using? Are you sure that you have configured the application pool for this application to use .NET 4.0?
Best Regards,
Martin Danko