Most probably you are running project built against ASP.NET 3.5 or 2.0 under ASP.NET 4.0. Please check the Framework version of .NET under which the application pool runs.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items