If GSX doesn't crash, that would clearly indicate the problem is nothing related to the Addon Manager or Couatl of their installation, otherwise GSX would surely not work, since it's the one that used them most estensively.
If you are getting crashes with other sceneries, it's possible some textures or AI textures are missing or corrupted in your FSX install, because a scenery attracts AI airplanes and sometimes might use default textures, while GSX use entirely its own textures so, it doesn't depends on missing/corrupted default textures.
We might have a better idea of the crash, if you go into the Windows Event Viewer, under the Admin Tools section of the Windows Control Panel. In the Application log, all your crashes are being recorded and, if you look for an FSX crash, its Details should give more information about what FSX module crashed.
