I'm using Symfony 3.0.2 with:
nginx version: nginx/1.4.6 (Ubuntu)
php version: 7.0.3-4+deb.sury.org~trusty+1

Everything works fine except when I try to enter the Symfony profiler where I get the following message:

Warning: session_write_close(): Failed to write session data (user). Please verify that the current setting of session.save_path is correct (/var/www/Symfony/app/../var/sessions/dev)

After that I get this error on every page. To be able to see regular pages again I just need to delete session cookie from my browser. But again, it works fine till I try to enter the Symfony profiler.

This error occurs only when I add to nginx config file this line:

fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;

However, when using php5 by adding the following line everything works fine:

fastcgi_pass unix:/var/run/php5-fpm.sock;

BTW. I have another project which is build on Symfony 3.0.1, and there is no such a problem.