Eric, thanks for replying.
Don't think I need to check for the Extendion Forms file as many other modifications I made inside Custom_JS_Lite are working fine, one of them being the addition of a top-level custom action. So also tag you mentioned obviously is working fine. And - yes - I verified the MenuID.
I'll give it a try the way you recommended (although I feel that I tried that already; but maybe something else was wrong then), and I'll be posting any results here.
At the moment this topic isn't as important since the customer in fact is quite happy seeing that button at the top-level of the relevant action bar.
Many thanks anyways,
Lothar