We've had an Alexa Custom Skill in production for a couple of years now. Recently (that last 3 weeks or so), we've had a very fast ramp-up in Account Linking error reports - with no changes to the skill or related services.
The error is presented as an HTTP 400 from https://skills-store.amazon.com/api/skill/link/...
We are using an authorization code flow process, and have found in our logs that the redirect_url from our login back back to Amazon looks good (hasn't changed) - including both state and code parameters. We also see the code being exchanged for an access token within the required timeframe.
After some extensive testing, we've found that the error only happens to users on Android phones through the Alexa App, or on Windows. Users on iOS and OS X do not see the issue.
Is anyone aware of recent changes that might introduce a regression like this?