I was receiving the "Invalid Formatetc Structure" with adding the RadMenu only.
I was able to correct it by using the solutions at the link below.
Resetting toolbox items has also helped many of them. (very simple solution)
Remove all project reference and add them one by one. Recompile and done!
This happend with WPF Q3, VS2008 trial, Vista, i believe it is a VS problem.