I was getting error saying "Control collection cannot be modified because control .... <% %>".
When I removed the RadAjaxPanel from the code the error was not there. What was wrong with my code? Or RadAjaxPanel just not support script inside panel?
I have found the following help article which explains more about this error.
Go through the section:Error message “The Controls collection cannot be modified because the control contains code blocks". General Troubleshooting