The default one is CTRL+Shift+F12, while CTRL+F12, by default, should be assigned to YouControl menus in FSDT airport, which will call GSX if the airport doesn't have a YouControl menu.
I'm not sure why you said you cannot change it using the KeyMapping menu. Are you getting errors ? If yes, which ones ?
In any case, you can just remove this file:
%APPDATA%\Virtuali\Keymapping.ini
to return to all default key for all our products.