There is a problem with the ini file. Below is all when starting fgfs.exe --launcher.
I removed the whole flightgear.org directory and then it worked. Then I copied the old FlightGear.ini file to the new FlightGear directory and I got an error on start: Fatal exception "Unknown variant org.flightgear.official.SenecaII". Yes, the Piper Seneca was the last plane I flew. So I deleted the recent-aircraft line and it went smoothly again.
After exiting FlightGear and starting it again, the same error showed up, of course that line had reappeared.
This happens for all extra installed aircraft, not for the standard Cessna 172. That aircraft gives a recent-aircraft line of file:///C:/Program Files/FlightGear 2016.4.2/data/Aircraft/c172p/c172p-set.xml, all other aircraft are in the form of package:org.flightgear.official.*
Something is definitly wrong there.