Sorry for the trouble indeed!
This error could be related to missing the RadControls for Silverlight designer assemblies.
Can you check the location of the Telerik assembly references in your project, navigate to that folder via Windows Explorer and check if there is a design subfolder there?
Are there any assemblies there?
Can you make a list of the assemblies in that folder and send them to us? (this can easily be done through the command prompt by navigating to the design folder and typing dir /s > FileList.txt)
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