Thanks for trying to help, but it seems this does not solve the issue.
Steven said: "just delete the directory".
You say ""delete the folder".
Now both of you don't really specify the directory/folder that is to be deleted.
If you mean the folder containing the music:
Well no

- but I did try renaming it, hoping MusicBee would then ask me if it can be deleted from the library, which it didn't. Both libraries continue to show up in the menu.
If you mean the folder containing the library:
Well yeah, in that case both libraries are gone and have to be added again by rescanning all files, which I tried to avoid. As I mentioned in my post, that is because of the huge amount of files and the very, very slow network.
Or am I missing something completely?
Cheers
B