The issue is caused by the TargetFolder property that you have set to your RadAsyncupload declaration. This property forces the control to automatically save the uploaded file to the specified folder. After saving the file to that folder, the temporary file in the raduploadtemp folder is immediately deleted. Within the postback handler you are trying to save the file again, but the control fails to find the temporary file (as it is already deleted) and reports the error observed.
To resolve the issue, simply remove the TargetFolder property:
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