The issue sounds quite unusual because the same was working properly at our on-line demo and there is no such known issue or breaking change in the control.
One possible workaround it to start using RadAsyncUpload that the control we recommend when working with Ajax as well.
If you want to preserve using RadUpload and you still can't fix the issue please share the code that will let us reproduce the issue locally and be more helpful.
the Telerik team
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