This is a known FSX SP2 problem: it's the result of the combination of using a scenery with custom runways made in FS9 style, with AI models originally made for FS9. Using the two together, will result in AI traffic disappearing, only in FSX SP2. Others FSX airports (like EGLL from Aerosoft) have the same problem, it's not related to LSZH in particular, it might impact all present and future FSX airports using custom runway textures. It doesn't happen using default traffic, since default AI are FSX native models.
There are two possible solutions:
- the scenery developer should remove FS9 scenery commands used to create customized runways, returning to default-looking runways. Unfortunately, there's no native FSX solution to create custom runway textures so, the scenery developer doesn't have much choice, except than offering a lower quality scenery.
- the AI developer should recompile its AI models using the FSX SDK. This would be of course the better solution, first because (opposite to scenery) it IS available to the airplane developer. And, it will also result in better looking and performing AI airplanes, without having to give anything away of the scenery.