I was able to reproduce the issue. It happens when Flash module is used to upload files. RadAsyncUpload control has four modules to upload files FileApi, Silverlight, Flash and IFrame. Sometimes the Flash and Silverlight plug-ins are causing exceptions which are beyond of our control. For that reason we recommend to stop their usage. Please set the DisablePlugins
property to true and test your page.
All the best,
the Telerik team
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