ASP.NET runtime error: The base class includes the field 'ScriptManager1', but its type (System.Web.UI.ScriptManager) is not compatible with the type of control (System.Web.UI.ScriptManager)

You will get this error when you have more then one AJAX Tool kit version install on the same computer. To solve this issue you need to add this code in you Web.config file.

<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<dependentAssembly>
<assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35" Culture="neutral"/>
<bindingRedirect oldVersion="1.0.61025.0" newVersion="3.5.0.0"/>
<publisherProfile apply="no"/>
</dependentAssembly>
</assemblyBinding>
</runtime>

This will tell the application at run time which verison of the AJAX to use.