How does PHP5-FPM send errors to nginx?
When your nginx config uses php_admin_value, php_admin_flag and php_flag it is overriding values in php.ini. Commenting out the nginx settings will leave the php.ini file in charge.
Take a look in the php.ini file at the settings for display_errors, display_startup_errors, error_reporting, html_errors and log_errors. There is probably a conflict there.