Do you do your tests on IIS or on a local host? If IIS, which is the version that you use? Please, paste your web.config here (excluding any passwords/usernames or connection strings) so that we can take a look at it. In fact, the point of interest is whether the RadUploadHttpModule is registered properly. The same applies to RadUploadHttpHandler. For more information on how to register properly the RadUploadHttpModule please take a look at the following link.
You need to follow the instructions for the Httphandler as well, just don't forget sufix it with axd extension instead of ashx.
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