Well, it seems that the problem you experience is a new one, which we haven't encountered so far. Is it possible for you to send me a screenshot of your GAC (Windows Explorer -> %windir%\assembly folder), navigated to any Telerik assemblies?
Can you please create a new website/web application and add some Telerik controls to the default page as well? Please, send us this small website/web application if it throws the design time problem for analysis. (For privacy reasons, please, use a formal Support Ticket to send us the file).
And the last suggestion (though you might have already done that), can you check the following folders for the existence of the Telerik.Web.UI
the Telerik team
Watch a video
on how to optimize your support resource searches and
check out more tips
on the blogs.
Follow the status of features or bugs in
and vote for them to affect their priority.