This won't work servers where IPV6 is not properly configured (most servers these days).
It would be nice to document this on the README.md
Create a troubleshooting section on the readme, starting with this:
Your server's IPV6 is not configured properly. Remove the line
from your server configuration blocks.
I've never ever experienced that on a Debian system. Yes I know there are people like there running stuff like CentOS and assorted lesser children of the Linux distros. In fact the config needs a serious updating of the docs, since there's plenty of undocumented stuff like the keepalive to upstreams and the fallback locations or the difference between TCP and UDS sockets for FCGI, or even more important, the caching setup. I'll updated it this weekend.