This error occured when using the http-server to run production builds of angular on a local machine.
Restart your computer, really :). The error means there is some process running already and you're probably here because you can't find it. Restart your machine will kill the process.
The solution is to clear your entire npm cache and folders (yeah, pretty lame). I could not found another answer (yet):
rm -rf node_modules npm cache clear --force npm install