Hello Umberto,
Thank you very much for your reply. I just arrived in Malta with the A320 from FSLabs for a turnaround flight back to Zurich. I parked the airplane at the stand, shut off the engines, set the parking brakes, the staircases arrived, I opended the doors and the passengers disembarked. After deboarding has been finished, the staircases disappeared and only arrived again when I started the boarding process. It might be that my process was wrong and I will check again after arriving in Zurich in about 2 hours. As it seems that the staircases should remain at the aircraft, I suppose that it's my fault in handling GSX or the aircraft.
Thanks again for your ongoing and excellent help and support.
Kind regards,
Daniel