The problem cannot possibly have anything to do with the scenery and, if anything, it could be an issue caused by your airplane.
You don't say which airplane you used but, assuming you are using an airplane that does progressive cargo/pax loading, like the Aerosoft Airbus for example, you must either:
- Call GSX *after* the airplane has finished its OWN passenger boarding simulation
OR
- Disable the "Estimate Passenger Number" option in the GSX Settings, and tell GSX how many passengers you have on board, so it won't have to estimate them from the payload, which is changing in real time.