Below are the links I followed in order to resolve
the mistery of the error
"UNKNOWN SERVER TAG" - related to asp:ScriptManager
Server Error in '/TestWeb' Application.
Description: An error occurred during the parsing of a resource required to service this request. Please review the following specific parse error details and modify your source file appropriately.
Parser Error Message: Unknown server tag 'asp:ScriptManager'.
Line 42: form id="Form1" runat="server"
Professional ASP.NET 2.0 AJAX
I took the Web.Config from the chapter 4 ( or 5 ? ) and everything was OK ,
problem solved !
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How to set path to the scriptreference in ScriptManager
MicrosoftAjax.js ,ScriptPath , ScriptManager