iTunes didn't allow me to choose the moved library file. The 'next' or 'open' button is inactive or doesn't work. Maybe I forgot to copy important files, but from this point there was no other option than to create a new folder. In older versions of iTunes, without this bug, iTunes worked fine with a new created folder on my external drive.. even with another name (not necessarily 'iTunes' since iTunes creates it's own folder automatically).
And honestly, I don't need the library files/playlists/ratings etc. I use iTunes just for purchasing and downloading music. All the other ballast (so to speak) is not significant for me. At least the default folder is changed successfully and I didn't encounter issues or warnings from iTunes when starting up. As far as I can tell, the problem is solved.