Such an issue normally happens when you've used a Telerik.Web.UI assembly built against a higher version of the .NET Framework than the TargetFramework
of the current project. This most often happens when a project,
targeting .NET Framework 3.5 has been created in Visual Studio 2010 and
it has been populated with the Telerik.Web.UI assembly in the Bin40
Please check TargetFramework setting of your project against the version of the Telerik.Web.UI assembly you are using.
Hope this helps,