The following ASP.NET controls are not compatible with partial-page updates, and are therefore not supported inside anUpdatePanel control:
Web Parts controls. For more information, see ASP.NET Web Parts Controls.
FileUpload controls when they are used to upload files as part of an asynchronous postback.
The Substitution control.
See a full list of controls that are not compatible with RadAjax for ASP.NET AJAX from the official Microsoft AJAX documentation here (scroll about halfway down the article.)).