Can you confirm if you are using the Ajax Control Toolkit on the page? If so, try the following:
1) If it is not the latest version or if your RadControls version is not the latest official, update to the latest ones. They work together seamlessly.
2) If both RadControls and the toolkit use latest versions, make sure that you specify the ClientIDMode for the page to be AutoID. This can be either done in the Page directive or in the web.config. More information about it is available here
All the best,
the Telerik team