I suggest you take a look at this forum thread which discusses a similar problem. As we are not acquainted with your exact scenario, we could not be sure what causes the issue. If the suggested forum thread cannot help you resolve the issue, please, open a support ticket and send us a test page which reproduces the problem so that we can test it on our side. For your convenience I have attached a screenshot with instructions how to open a support ticket. I would also like to ask you explain in more details what version of the Telerik.Web.UI dll you used when the problem did not exist and what is the version you use now. Is the setup of your page exactly the same as the one before the problem appears or you have made some modification?
the Telerik team
Check out Telerik Trainer, the state of the art learning tool for Telerik products.