Could it be that you have both RadControls for Silverlight Q3 2009
(or a service pack) and RadControls for Silverlight Q1 2010
installed together on the machine? It seems that for some reason the VSExtensions package searched a file in the wrong location. If that true, can you please, try removing the older RadControls version? If you need both, please, rerun the installer of the older version, enter Custom Installation mode and uncheck the Visual Studio Extensions feature.
Looking forward to your reply,
the Telerik team
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