Currently when the RadButton is focused and the Enter key is pressed, the server-side event OnClick is not triggered. When the Enter key is pressed again, the server-side event is executed.

The issue is reproduced when a dialog is opened in OnClientClicking event.

The workaround is to use

-Either the OnClientClicked event
-OR trigger the server click of the RadButton in the OnClientClicking event:
		<script type="text/javascript">
			function OnClientClicking1(sender, args) {

				alert("Client-click triggered");
				sender.click();
			}
		</script>