There are some incompatibilities between our Silverlight 3 assemblies and the Silverlight 4 runtime, hence we released a separate Silverlight 4 build of RadControls for Silverlight. Do you use our Silverlight 4 assemblies in the upgraded application? The version number should end with 1040 for Silverlight 4 and 1030 for Silverlight 3. It would be of great help if you open a new support ticket and send us the upgraded application.
the Telerik team
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