@Mcbain
You don’t have to change your Home Assistant url; you just need to specify the https://my.home-assistant.io/redirect/oauth entry in the Spotify Developer Application settings “Redirect URIs” entry. You need to specify the entry exactly as it is spelled, and without the ending slash. This is an internal url that Home Assistant uses to intercept the OAuth2 request redirect result.
@Mcbain
You might try going to the Application Credentials settings (under Settings \ Devices, click on 3 vertical dots menu in upper right and select Application Credentials) and see if it created any oauth2 application credentials related to spotify. If so, delete them.
You might also open the Spotify Web Player and logout of Spotify. That might at least force you into a logon screen the next time you try.
Can you supply a screen capture of your Spotify Developer Application settings? Just want to verify your Redirect URI’s are correct, and that you have the Web API option checked.
@Mcbain
Not necessary on the coffee, glad I could help. I’ve received a lot of help on this forum, and I’m just paying it forward. Glad you got it resolved.
If you’re interested in Spotify, check out the SpotifyPlus Integration; it can do everything that the HA Spotify integration can do, and it supports both Sonos and Google Chromecast devices (Spotcast not required). You can also use the same Spotify developer account Client ID / Secret that you created for the HA Spotify integration if you want. You can run them side-by-side to compare features too.
There is also a SpotifyPlus Card user-interface that contains a media player, and allows you to manage your Spotify favorites as well as search the Spotify catalog. You have to install the SpotifyPlus integration first though, and get it configured before you can use the SpotifyPlus Card.