I figured out my problem, I'm just posting this in case it helps someone else.

I'm not sure if it matters, but I am using the current version of the spotify-ruby gem, and will make this same report there, although I think there is something to learn here for Spotify about how errors could be better reported.

Using the gem, I am obtaining an URL like this:

url = @accounts.authorize_url(scope: scopes)

appears to be a correct oauth authorization URL, but loading it in the browser always results in the subject error (embedded below).

This is an example of the URL:

https://accounts.spotify.com/oauth/authorize?client_id=somesecret&redirect_uri=someEncodedURL&respon...

The problem turned out to be that the "/oauth" part of the URL was incorrect. If I removed that portion, the page loaded fine with the permission flow.

url.gsub!(/\/oauth/, '')

So it looks like the Error page that loads is really a 404 page. It would have saved me some time, and probably a lot of people some time, if that page was more clearly a 404 page and not a vague "something went wrong".