Most probably there is some inconsistency in the telerik assemblies you used.
Will you verify the telerik assemblies are all with the same version? It could be great if you may share whether the exception is reproduceable only in Expression Blend Environment or not.
Also you may take a look at the following thread with an issue related to the yours, follow this link.
If this does not help feel free to submit a new support ticket where you may attach your application.
We may take a look at it and provide you with an appropriate solution.
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