They changed the API to not allow localhost anymore, have to use 127.0.0.1 instead:
https://developer.spotify.com/blog/2025-02-12-increasing-the-security-requirements-for-integrating-with-spotify
I've fixed this now https://github.com/strawberrymusicplayer/strawberry/commit/9ed89afdb2063baaac694f59e7a97d27871e7a01