I noticed that you send us a support ticket as well. Here I will paste my answer from the ticket in case someone else faces the same or an issue similar to this one:
Unfortunately the information that you provide is not sufficient to determine what is causing the problematic behavior. Therefore I would suggest that you set the DisablePlugins="true" property of the RadAsyncUpload and check if you still get the same behavior. In general by doing so, will help us determine if the behavior is due to the control itself or due to any of the used plug-ins. Can you also send us the error messages that you get when you experience the failure, so we could get a clearer idea of what might be causing the problematic behavior?
the Telerik team
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