"Broken pipe" between Nginx and PHP-FPM
An year and a half later ...
Looks like it's xdebug: https://bugs.xdebug.org/bug_view_page.php?bug_id=00001401
If you turn debugging on (so something is listening on port 9000), the error goes away.
(Found this while searching for the same problem, so I thought I'd share)