I have just upgraded from VS2008 to VS2010. The first time that I ran my site in Debug I got a build error
Error 1 Unknown server tag 'asp:UpdatePanel'. D:\DNN\Development_Website\admin\ControlPanel\AddModule.ascx 5
I also got 
Error 106 Reference required to assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' containing the implemented interface 'System.Web.UI.IScriptControl'. Add one to your project. D:\DNN\Development_Website\admin\ControlPanel\AddModule.ascx 1 1 D:\DNN\Development_Website\
In web.config I have the following entry under <assemblies>
<add assembly="System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
All references in web.config are to Framework 4.0, but only the AddModules.ascx throws an error

I've successfully upgraded to 5.6.2 but when I run debug in VS2010 with .NET Framework 4, I still get the error.



If I open admin/ControlPanel/AddModule.aspx, the control declaration is underlined and the tip is 'Reference required to assembly System.Web.Extensions, Version=3.5.0.0 .......



Presumably when my site was upgraded to use .NET Framework 4.0.0 this file seems to have been missed. Doesn't look like a download or file corruption problem as error still there after upgrade to 5.6.2, must be a bug in the core.