Well - here the News from Germany.
After the installation of KJFK failed, I uninstalled first only the Airport, without couatl and addonmanager.
Result: the FSX didn't come up again.
So I started from the beginning, uninstalled Geneva and Zurich, too and started the new installation with Zurich like we did before.
Same problem - after the Zurich installation (and after reinstallation of the addonmanager) the FSX didn't come up again.
No chance for a screenshot, sim.ini could not run.
So, I reinstalled the former downloaded VisualC-Files again, installed Zurich and the Addon-Manager and...
....it works again, the same with Geneva.
I suppose, the installation file of KJFK must change anything in the system during installation, maybe the C++-parts.
I cannot try it the same again, I stop and use only Zurich and Geneva. Please check this and try to solve it.
I'm really tired of testing and trying since 3 days now - and it only should be a hobby....
Attached the cmd-file of the actual situation....
Martin