I have tested the two Drag and Drop samples on the WPF demos but I did not get such an error. Have you followed the code samples there?
You have stated that you have a .zip file with the sample, but I cannot find it attached. Would you please send the project to us so that we could further investigate what is going wrong.
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>