That the error cannot happen, basically, unless you haven't installed the latest GSX full version OR theres something wrong in your Simobject folder, which prevents GSX to see its own objects there, but it didn't appear from your FSX.CFG file.
Maybe, but that's just an unproven theory, you have too many objects in the Simobject folder ( airplane + AI + everything else ) and there's an undocumented limit on the number of objects that FSX can handle, that we are discovering right now.
If you have AI airplanes from a 3rd party AI traffic package in a separate folder, referenced in the FSX.CFG by the SimobjectsPath lines, if it has its separate entry, usually the 6th line (FSX defaults with 5 lines), try to comment it with a ; in front of it, and see if it makes any difference. Of course, don't comment the Simobjects\Misc line, because there's the one GSX needs.