AJAX: Unknown server tag 'asp:ScriptManager'

 

add the <Pages>...</Pages> given in the frist post above to <System.Web> element in the web.config.
something like this:

..
<System.Web>


<pages>

<controls>

<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />

</controls>

</pages>
</System.Web>