I forgot that Valid() wouldn't fire if checkbox status didn't change. Try

* WHEN
DO CASE
CASE EMPTY(ON("KEY", "ENTER"))
	ON KEY LABEL ENTER KEYBOARD "{TAB}" CLEAR
CASE LASTKEY() = 13
	ON KEY LABEL ENTER
ENDCASE	
...

>Because I'm trying it and the ENTER key does not work in the screen after the WHEN turns it off.
>