@Laubender, this one takes me from the last opened doc to the 3th doc (if 5 docs are open) and then to the first, then the 4th, the 2nd and last again. your script makes sense but actes strange here.
Nevertheless both scripts are working fine when double click them but not using the shortcuts I want.
Now I have found out that when I assign CMD-F13 and CMD-F15 to those scripts, they work, even with open files.
So the main issue is still, why is Indesign ignoring the CTRL-TAB shortcut?