Symfony 3.1.5 Warning: SessionHandler::read(): Session data file is not created by your uid
Editing config/config.yml and replacing save_path: "%kernel.root_dir%/../var/sessions/%kernel.environment%" with save_path: /var/lib/php/sessions fixed the error.
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AVOID change prod configuration for troubles that only occurs in development environment
Had the Same trouble when using Vagrant (PHP-FPM 7 + Nginx) with a sync folder (/vagrant ~ default)
Solved just put
session: save_path: '/tmp'on config_dev.yml inside framework entry
Just choose some place that Webserver and PHP can read/write without permission issues.