Hi!

It seems Symfony dosn't change the session storage path %kernel.root_dir%/../var/sessions/%kernel.environment%, when checking app_dev.php/_profiler/phpinfo it returns /var/lib/php/session.

This lines solves the problem temporarily:
session_save_path(__DIR__ . "/../var/session"); in app.php and app_dev.php

I got the exception when authenticating because Apache dosn't have write access to the default PHP session storage path.

I'm running Symfony 3.0.0 with PHP 7.0.0.

The project that I noticed the issue on:
https://bitbucket.org/Richardh87/messenger