I am still not able to reproduce the behavior you are experiencing. Could you try to clean the cache of the browser and check if the issue persists in different browsers as well? It would be great if you could test if it is reproducible on another machine. You could test also if you can reproduce the behavior using the local project with another version of the controls.
The exception you are getting is very general and could be thrown in different scenarios. Have you tried to enable the CLR and Silverlight exceptions? You could find this option in the Debug -> Exceptions menu in Visual Studio. Can you see the stack trace of the exception?
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