Hi,
I know extremely little about php, or sysadmin in general, and only randomly made this work, so I don't know exactly what the problem is.
But on my debian 9 system, I had to modify the suggested nginx configuration here:

https://freshrss.github.io/FreshRSS/en/admins/02_Installation.html

to set this lock file to /var/run/php/php7.0-fpm.sock instead of /var/run/php5-fpm.sock