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Recently we updated the upload control. We suppose that the issue relates to this update. However there should not be any errors if you re-reference both the client application and server-side upload handler.
The other possible source is mixing a Silverlight 4 application with RadControls for Silverlight 3 and vice versa. Please check that version of the RadControls' assemblies coincides with the application version.
Please try the suggested points and let us know if you need further assistance.
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