No, I don´t have it installed.
I now found out it always works in Sedona. At Borkum, which is part of the East Frisian Islands Addon it never works. I also saw the available GPU memory fluctuate between 4355MB and 5284MB. Is that normal?
I guess it has to do with the GPU memory being full that it doesn´t work at some places. Unfortunately when it doesn´t work, the indication in developer mode and FPS shown freezes. Maybe addon sceneries are too memory heavy for my graphics card. As the settings are pretty low I guessed 6GB should be enough.