Error Pages | Heroku Dev Center
https://devcenter.heroku.com/articles/error-pages09.03.2020 · SSL Heroku’s HTTP router serves unstyled HTML with HTTP status code 503 (Service Unavailable) when your app encounters a system-level error, or while maintenance mode is enabled. Other errors, such as application errors (a 404 or 500), will display your application’s error page and not the Heroku error page.