"Bad gateway" with nginx, PHP-FPM using FreeBSD socket

After updating a FreeBSD jail I got the dreaded "502 Bad gateway" error from nginx, here's how I fixed it:

Apparently there is a bug in the PHP version shipped with FreeBSD; ServerFault user MafiaInc kindly provided the solution:

1. Edit the file /usr/local/etc/php-fpm.conf

2. Note the values of the user/group lines, e.g.:

   user = www  
   group = www
   

3. Add these lines with the same values as the user/group lines:

   listen.owner = www  
   listen.group = www
   

4. Restart PHP-FPM using this command:

   service php-fpm restart