We had an issue within the Telerik.Windows.Controsl.Diagrams.VisualStudio.Design.5.0.dll.
As this assembly is designed to work in VS2012
, you can remove it from the design folder thus getting over the exception.
Also, as the issue is already fixed, you can download the Q3 2012 SP
release that is due by the end of the week, and test your solution with it.
Let us know if that helps.
the Telerik team