@projectmoon @amaurydbz found it. Spotify is not allowing using my keys to login into your account, which means that you'll need to login into Spotify Developers, generate a pair of keys and bring those to the app.
That's why it works for me, but not for you. It uses my keys.
As the app does not have a way to do that yet, I unpublished it. When I add a way to do it, I'll let you know and publish it back.
Thanks for the feedback!
Link for reference: https://developer.spotify.com/blog/2026-02-06-update-on-developer-access-and-platform-security