Hi Wissam,
This particular error is not specific to Telerik RadControls for ASP.NET AJAX. If you've used the Microsoft ASP.NET AJAX UpdatePanel control, there's a good chance you've hit the "Sys.WebForms.PageRequestManagerParserErrorException" error. Please have a look at
this article to know more about such an error and how it can be avoided in your application.
Thanks,
Princy.