Does anyone know, why something like this - see screenshots attached - happens and how to solve this issue?It happens not only to touchdown effects, but also to the wake effects behind boats/float planes, contrails, engine smoke, etc.thanks a lot in advance

It seems that some textures for the effcts are missing. Just make a quick repair with the FSX CD.

Why run a repair and possibly mess a lot of things up? Just go into the aircraft.cfg file and look at what effect it's trying to call, then try and track it down in your fsx folders.