I have now spent the last few hours deleting all the textures and copying them again following your instructions.

Unfortunately, I cannot find the textures or can't select them. There are always whole packages, e.g. with 40 or 50 models. When I want to change the texture, only one model from the package appears. How do I select one from all the textures? Or where do I find them? Textures were copied into the library. Where are they then?