I know this is a very old thread but I experienced this same issue and was unable to find any other reference to it on the web. So, I'm posting my solution in case it might help someone else, some day.

It seemed like any admin screen that contains any of the Telerik controls would not work, at all. At first glance, the problem seemed to be with the Telerik.Web.UI.WebResource.axd which was serving up some javascript. The web server returned a 404 for the URL so no javascript file was downloaded. That caused a js error so the rest of the js didn't load.

I couldn't edit content in any HTML Module. I couldn't use the Page Management screen. It was happening on only one webserver. I was not able to reproduce the behavior on any other server.

After a lot of research, trial and error, we finally figured out that it was caused by an IIS setting that truncated long query strings. The server administrators modified the allowed querystring length setting under Request Filtering / Edit Features in IIS in order to satisfy a PCI compliance audit. They just turned it down a little too far. Once that value was increased, all the RAD controls in the Admin screens started working again.

Unfortunately, I don't know what the value was set to but If you experience this problem, take a look at or ask the sys admins about the querystring length setting in IIS.