If I am correct, you are using the RC version of Silverlight 4 (I can't remember what was its runtime version), which most probably is the reason for the problem. I would recommend upgrading to the RTW Silverlight 4 (version 4.0.50401.0) and then clearing the browsers cache. Some of our colleagues had the same problem and it was "cured" after installing the new Silverlight and several cache clears.
All the best,
the Telerik team
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