You can include whole folders in libraries using Copy INCLUDEINLIBRARY=documents. The libraries are a virtual location,
whereto real copying is not possible.
That isn't true; each library has a default folder where files are copied to if the destination directory isn't implied in some other way (e.g. being inside a folder below a library).
Sorry. I assumed, that it´s not the case, because including large film & music collections then logically causes a big consumption of space, doubling the data. I wouldn´t have thought that.
Sorry, please try and you will see that you are wrong on that.
Please call your Winodws Explorer, right-click a library (in the left hand pane) and click on Properties. A new window pop-up showing you options to add folders and customize the library. There is a button "Set save location" to set the standard folder for "real coping" files in the library.
(1) try to copy in the main area (on the right side, details view for "Libraries"), I can't copy the file because "paste" is greyed out.
(2) If I try to copy into the folder tree (on the left side, "Libraries" subfolders), I get this error message "An error occurred: The operation is not supported by this VFS".
The strange thing is that coping via drag-and-drop ("Copy Here") on both sides is no problem at all.
Yes, but please go one level above your current directory, mark "Wallpaper" with right-click itself and try to select "Paste"... then it should be greyed out on your DO too (1). And can you reproduce this error message (2)?
In contrast to that both should work fine via drag-and-drop.