While upgrading from .net 3.5 to .net 4.0 with Visual Studio 2010 we encountered an nasty issue.
Method 'get_EnableCdn' in type 'System.Web.UI.ScriptManager' from assembly 'System.Web.Extensions, Version=188.8.131.52, Culture=neutral, PublicKeyToken=31bf3856ad364e35' does not have an implementation.
With the help from Google/MSDN its somewhat clear the third party references to the old System.Web.Extensions were the rootcause. Here is one example of an issue.
But we dont use the third party libraries refered in the above link, but instead we used a old version of dll from Microsoft itself which is Microsoft.Web.Preview.dll and the History control inside the Library. It looks like there is no updates for this library in .net 4.0. But instead we used the History feature intergrated in scriptmanager itself. Just remove the Microsoft.Web.Preview.dll once the changes are complete and now you will see the issue resolved.
fullmeals dev team