Im also having the same error message as the OP! open.spotify and play.spotify does not work. They were working a few days ago! My free trial ends in few days and I was willing to be a paying subscriber but this is a deal breaker! I dont wanna switch to a new browser nor download the desktop app! Spotify needs to fix this!
I downloded the app and now it works. I really don't want to be downloading any apps though, so if i find a way to make it work on safari, I'll go back to using it on safari. Other than that, i'll most likely end up listening to some other alternative website like deezer or soundcloud.
I don't wanna move away from Spotify. I have built up playlists since 2011. And I think they have more content than Apple Music or google play. And I don't like google musics app interface too bright and hard to navigate. Not sure yet about Apple Music but if there is a way to import my playlists to them automatically I might give them a try.