Issue with Ember Server: Port 4200 is already in use when no application runs on 4200

I'm trying to make my ember application run but whenever I start it with ember serve -e local I get:

Port 4200 is already in use.

Which is weird because no application is running on 4200... I tried to open a dummy HTTP server on 4200 with http-server and it works just fine... Useless to say that I already:

• Restarted my computer.
• Removed the node_modules.
• Remove the ember tmp directory.
• Disabled my firewall.
• Tried with disabled wi-fi.

Any thoughts?

I'm running on masOS High Sierra 10.13.6 with the following versions:

• node: 8.11.3
• ember-cli: 2.18.2
• npm: 5.6.0 (also tried with 6.3.0)

Also useful to know:

• I have other ember applications running on my computer just fine.
• I started to have this problem only recently.
• In the past, macOS would keep asking me about authorizing incoming traffic whenever an app was opened, but now it stopped asking me.

I suspect the problem is coming from the operating system rather than the app in itself, but I can't wrap my head around the issue.

Any help is greatly appreciated!