Today I am also experiencing the same error when trying to add the curated lists from UKF to my library.
What I tried to do in iTunes was:
- Searching for UKF
- Select curator to see the page
- Select playlist
- Click on "+ADD" button
Then I get the message "Adding <playlist name here> to Library" Due to error 500.
If I try to add the lists in iOS it seems it works, after tapping on "subscribe" I can see the lists under "playlists" in my library.
Any hint how to debug or fix the issue?