I have the problem that no customized parking position is loaded on startup.
If with "startup", you mean when you start a flight there from the Main Menu, it's not a problem, is completely normal, because when you start a flight, the vehicles you see are NOT the "real" GSX vehicles ( those will only appear after you call GSX ), they look like GSX vehicles, because their models have been replaced by GSX, but they are default Ground Vehicles, which are placed by the simulator according to their own rules, not by GSX.
When you start GSX and choose a service, those vehicles will be replaced by proper GSX vehicles, which will use GSX rules, including your customization.