In the Q3 2015 SP1 release the server-side OnClick event of RadButton is not raised when the following conditions are met:
- The RenderMode of the button is set to Lightweight.
- The .aspx page is opened under Chrome or Edge browser.
- The button triggers an AJAX request.
- The text of the button is clicked.
The issue will be fixed for the official Q1 2016 release and the 2015.3.1124 internal build.
For the time being you can use one of the following resolutions:
- Set the UseSubmitBehavior property of the button to false