Heroku/Rails/Devise: The change you wanted was rejected

Turns out the issue (at least for me) was being caused by the Blur / DoNotTrackPlus extension for Firefox. I had previously disabled it, but that had no effect. When I removed it, however, the problem went away.

Hope this helps someone in the future.

This turned out to be a problem with Cloudflare for me. I missed the following log line when reviewing the error:

HTTP Origin header (https://test-app.my.app) didn't match request.base_url (http://test-app.my.app)

I had the Flexible SSL mode enabled for my domain. This needs to be set to Full SSL to ensure that https:// is forced when Cloudflare communicates with your app.