This is a known issue for the version (RadControls for ASP.NET AJAX Q3 2008) used in Sitefinity 3.6 Hotfix. The latest version of the RadControls will be included in Sitefinity in the upcoming service pack (scheduled for the middle of April). This means that the fix of the described issue will be available in Sitefinity SP1. Please note that we do not recommend clients to upgrade RadControls on their own but rather to wait for the next Sitefinity version where the latest RadControls are added.
the Telerik team
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