Thanks for the links, useful. One thing they could have added is a lot like a final note on RadUploadProgressHandler.ashx, is that depending on security configuration of the site you need:
in the configuration element of Web.config. The login page needed MicrosoftAjax.js from the Telerik.Web.UI.WebResource.axd but the server was denying access wanting forms authentication, so you got to them same page (login) and a returnurl was set somewhere to the resource. This was not transparent at all, should have looked at the server logs earlier.