This is a breaking change in .net 4.0
which causes additional validation for the values in the parameters page. We will provide a fix for this change in the next release or internal build. For the time being you can solve the problem by adding following entry in the web.config file:
You can find additional information on the change in the following blog post: RequestValidation Changes in ASP.NET 4.0
All the best,
the Telerik team
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