Can you verify that you are using the latest version 2010.1.415 of RadControls for ASP.NET AJAX in your project? If not, migrate to it following the instructions from this KB article.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking system and vote to affect the priority of the items.