When starting Rails in using foreman like:

Procfile

web: rails server

It defaults to taking port 5000. On newer osx versions this port is now taken by MAC OSX though, so it won't start (or in some cases do start but the requests go to the airport process): https://www.reddit.com/r/webdev/comments/qg8yt9/apple_took_over_port_5000_in_the_latest_macos/

Of course you can fix this by specifying a -p but I guess the default should be available.