Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.

Server Error in '/Imagesample1-ModalPopup' Application.

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Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.TypeLoadException: Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[TypeLoadException: Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.] AjaxControlToolkit.ToolkitScriptManager.OnResolveScriptReference(ScriptReferenceEventArgs e) in c:\TeamCity\buildAgent\work\86982cda3515e83b\Server\AjaxControlToolkit\ToolkitScriptManager\ToolkitScriptManager.cs:276 System.Web.UI.ScriptManager.RegisterScripts() +261 System.Web.UI.ScriptManager.OnPagePreRenderComplete(Object sender, EventArgs e) +117 System.Web.UI.Page.OnPreRenderComplete(EventArgs e) +2063008 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2247

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Version Information: Microsoft .NET Framework Version:2.0.50727.1433; ASP.NET Version:2.0.50727.1433 
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Two Reasons To Cause This Error:

1.Install Framework 3.5 SP1 (Service Pack 1)
http://www.microsoft.com/en-us/download/confirmation.aspx?id=22

2. If you are using Ajax Control Toolkit then use ToolkitScriptManager

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

<cc1:ToolkitScriptManager ID="ScriptManager1" runat="server"></cc1:ToolkitScriptManager >

or else use

<asp:ScriptManager ID="ScriptManager1" runat="server" > </asp:ScriptManager>

Download Ajax Control Toolkit And Add To Visual Studio ToolBox
http://www.asp.net/ajaxlibrary/act.ashx