I'm not sure why I should ask Aerosoft, because so far this is the only airport this is becoming a problem
That's because, in you FIRST post, you indicated a fairly precise message, which is NOT part of GSX and I suspect it's part of the airplane, the one that says "FOLLOW GSX INSTRUCTIONS". To this post, I replied to check with Aerosoft, because that message surely doesn't come from GSX.
Then, after my reply, you posted ANOTHER screenshot, showing the normal GSX Parking menu. From your screenshot, is not very clear where you are located. Are you supposed to be inside a parking ? If yes, and you still see the GSX parking menu, then the problem is you have an AFCAD conflict, likely caused by another scenery, so all parking positions read by GSX are not the one correct for the scenery.
With the GSX "customize airport positions", you can check the name of the AFCAD used by GSX, which might give you some hints about which scenery is causing the conflict.