Heroku uses the referrer URL to point at the repo that should be deployed; from this page that includes part of a path that breaks the deployment (specifically /blob/master/docs/Running-Mastodon/Heroku-guide.md).
I've replaced the vanilla address with one that includes a specific reference to the root of the repo
work flawlessly was a nightmare). WARNING: This commit makes the web UI connect to the streaming API instead
of ActionCable like before. This means that if you are upgrading, you should set that up beforehand.
Valentin Lorentz