I'm not sure whether this is a real bug or just an error case that could be handled more gracefully.
After editing a playlist with emacs on my PC, I loaded it in library view, and got a listing of songs. However, when I tried to add it to the playlist builder, it churned for quite a while and then gave up saying an error occurred.
I suppose the problem was due to duplicates, because after removing duplicates from the list, everything went fine.