They never did follow it up, despite many exchanges from me. However, I resolved it myself to a degree. I deleted all of the playlist that were created by Spotify and stopped following artists. Essentially I now only have the playlists that I have created. I did this on my PC, then logged out and back in on my wi-fi player and the playlists loaded up immediately, like they used to. The only issue I have now (apart from if I add a Spotify playlist it reverts to the problem) is that when I add any new playlists or songs/albums to the lists on my PC, I have to log in and out of my Spotify account on my wi-fi player for it to refresh and be available on the wi-fi player. It's not ideal but I can live with that.