Connection refused with local development proxy

I use koa-proxies for local development. Recently, all of a sudden the proxy stopped working, i.d. the routes where correctly mapped (add logs:true to config), but when I called the service with the proxied port, I only got 503 from the backend.
After tearing my hair out, I check /etc/hosts file and removed the ip6 localhost entry (::1 localhost). After that I worked fine.
I spotted that when I doing a curl with -vvv option on. Still don’t know when this got added. I installed cntlm for testing purposes. Maybe that inserted this entry, as this is the only proxy related thing I did around that time.