Since the most recent update, I can only add songs to the most recent playlists I've created but not older ones. If I use the right-click drop down list, it only shows my most recent playlists and does not allow me to add to older ones.
I have the same problem. But if I select an older playlist, let it load, and then try to add the track that is playing, the playlist now appears on the right click list of playlists which can be added to... so I spent a minute keying through all of my lists, and now they are back. Weird, but works.