Initializing the GUI from a different thread

Dear all,
I have a question concerning setting up a multi threaded application.
We have an application (https://github.com/eclipse/sumo) which works 
well if we initialize the FXApp and the main window from the main thread 
but it I use another thread to initialize the GUI it does not work any 
longer. The problem is that the application spawns more threads which 
communicate with the main window using an event queue (from the FXEX 
package: 
https://github.com/eclipse/sumo/blob/main/src/utils/foxtools/FXThreadEvent.h) 
but the main windows does not seem to get the signals any longer if we 
start it from the secondary thread. The problem occurs only on Windows, 
on Linux it works fine.

We are using FOX 1.6.57 and any idea is welcome.

Best regards,
Michael