We plan to re-introduce Wednesday's code change again later this week with a clearer error message. If your app uses the authorization code flow then please make sure that, when redirecting users to the /authorize endpoint, your app only uses query parameters that are documented in Spotify's OAuth guide. Adding extra query parameters (e.g. code_challenge or code_challenge_method) could cause your app to have compatibility problems again starting on Thursday.
I hope this helps to clear things up - if you have any questions, feel free to ask them here in the thread 🙂
Thank you for a heads up. I must admit this worries me since I have never used the authorization flow any other way than documented in your official docs. I also received an error message on the /api/token endpoint and it seems like you only mention changes in the /authorize endpoint?
Is there any way that I could test my app with the newest endpoints before an official release? Since I would be very happy to avoid my app breaking in production again. I hope this is understandable.
I would suggest testing your app's installation flow as a new user and stopping after you are redirected to accounts.spotify.com/authorize. Take a look at the query parameters in the URL. You should see client_id, response_type, redirect_uri, and possibly state, scope, and/or show_dialog as described in the OAuth guide.
If there are extra query parameters in the URL (for example code_challenge) then, starting on Thursday, you could receive an error when your app tries to exchange the resulting code for an access token. Hope this helps!