I ran the IF setup again. strange thing: per default, it showed the correct V5 folder destination.
first time I clicked on "!", the box was blank! 2nd, 3rd, 4th etc. time it went back to showing the correct V5 folder. I selected Update now. NOW IT WORKS!
just to be sure, I checked the log and V4 files folder. nothing there, no error messages.
Thanks for the hints!
P.S.: Next thing I have to fix with Lockheed Martin is: my kneeboard only shows blank pages... but since that is true for default missions as well, I think that is no IF issue...