YES! That's exactly what was going on. Thank you very much for the link! Did everything it was recommended on the thread. Should it not work I'll come back here tu update the issue, but I think it should be okay now.
@vicarious I tested this here with "Start from the beginning with start page" and "Open a specific page or a page group", and in both ways, closing the window on the secondary screen didn't close the one on my main monitor.
Did you have the "GX Corner on the sidebar" enabled?