We managed to reproduce the problem on one of our machine and on the rest the correct error screen was shown. Further investigation showed that this is a problem with the server, which was enable to server error page, despite that the client was redirected there. Now, please note that this behavior is implemented by the ASP.NET runtime and it is the same whether you use FileUpload or RadUpload. This is the first time we receive such report so we will try to troubleshoot it further. Meanwhile, could you please confirm that you reproduce the same problem on different server than our online demos?
All the best,
the Telerik team
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