Developer only worked on one procedure in that APP and fortunately recalled the Window Previewer showed an error. I love a good Previewer puzzle so I tried it and saw below message:
The Preview shows with the below Caution icon that makes it obvious:
Traced that down to the below code with ICON('A')
Which was accidentally entered in the button properties:
So in the case of a “Source file XXXX.clw not found” error try searching source for ('XXXX'). I tested with IMAGE('A') and received the same error about A.CLW not found. Prior Format.CLW post suggested search for Module(‘X’) could be the problem so that fits the search for ('X'). I would also try doing a Window Preview on the recently modified procedure Windows.