dear jroberts, thank you for your reply. I do have the latest version of X-plane (PB 15) but the problem still exists, not only with B 747-800i but also the 737. The reverse function after touch down is a complete mess, whether I operate it by the "silver slider" (Saitek x52) or any button or key: sometimes it cannot be pushed back to normal, sometimes – after several attempts – it does go back but the plane keeps going backwards or cannot be moved into any direction. If the problem is the Saitek x52 with XP 11 it should really be fixed as soon as possible. In XP 10 flying the Airbus 380 there was no problem!
Anyhow, since I am a beginner, the problem could be somewhere else and may perhaps apply to the Boeing series only or any other feature within XP 11.