Ember serve: every port is in use

When I specify no port or set the port explicitly, I always see:

ember serve --port 4207
Port 4207 is already in use.

It doesn't seem to matter which port I pick.

When I start with port 0 (let the cli pick an available port) it starts on on something around 30000~40000.

I am on a Mac. I just upgraded my ember-cli to 3.12.0

What is wrong here?