Yesterday I was working on a project which had the AJAX Extensions in use. It’s primarily a .NET 2.0 application and I had .NET Framework 3.5 SP 1 installed on my system. It was working normally and all of a sudden I got this error that: Unknown server tag ‘asp:ScriptManager’
It was strange because every thing was working and I had the reference to the AjaxControlToolkit.dll.
I found out that by adding the section below to the <system.web> node in the web.config solves the problem:-
<add tagPrefix=”asp” namespace=”System.Web.UI” assembly=”System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35″ />