Now the problem, I have: When I installed Zürich into P3D V4.2, I saw the entry in the scenery library (P3D -> World -> Scenery library), also the P3d Scenery library
I'm not sure what do you mean with "the scenery library", or the "p3d scenery library", as if they were two different things. Maybe you wanted to say Zurich appeared BOTH the the P3D->World->Scenery Library (that's the P3D Scenery Library) AND in the Options->Add-ons (that's the list of the Addons) ? If yes, this is entirely normal.
The only entry was: Zürich. All other entries had disappeared.
What do you mean with "all other entries" ? Other FSDT sceneries, or ALL the other entries, including the default scenery ? I'm sorry, but if the other entries disappeared, it's nothing that has anything to do with any of our installer since, as I've said, we DO NOT TOUCH THE SCENERY.CFG!!!
However, the scenery.cfg (C:/Program data/Lockeed Martin/P3D v4/..) contained also all the other entries.
Yes, you might see them but, it's possible the file has an error, or has been corrupted by ANOTHER installer (we DO NOT TOUCH IT!!), maybe one that doesn't recognize the different file encoding which is now Unicode (compared to P3D3 and FSX Ansi), so it has destroyed the file, and the sim is not reading it correctly,
Don't you think that the scenery library in P3D (not the scenery.cfg) should contain all those previously installed sceneries, so you could edit them, or change priorities?
I think you haven't understood what scenery library means in P3D4. The List you see in the sim should contain BOTH sceneries installed in the proper P3D4-way (like we do), with the add-on.xml, and all of these will appear greyed out, but ALSO scenery areas listed in the scenery.cfg.
This is the normal situation and this is now it always works when installing any of our sceneries: we DO NOT touch the scenery.cfg, but our scenery areas will appear in the Scenery Library, greyed out, together to the other areas coming from the scenery.cfg, which we DO NOT TOUCH.
So, if you have any problems with OTHER sceneries which are supposed to be in the scenery.cfg, but are missing, the only thing sure, it's your issues is entirely unrelated to our software or installers, you probably have a problem with the scenery.cfg which, as I've said, none of our P3D4 installers ever touches.