What is the reason for the 500th error in Symfony on an external server?

Question

symfony

Problem: when going to a site with Symfony 2.8.3, it correctly takes the user to the login page /login, but after entering the data correctly, when redirecting to the main one - "500 Internal Server Error". Nothing is written to the error log.
At the same time, on a PHP 7.0 server with CGI (without CGI scripts, the php version drops to 5.4, and the application does not work).
Before that, there was basic authentication, which did not work even with the crutch https://github.com/symfony/symfony/issues/12508
Authentication itself works on the local server. Made according to the manual ( symfony.com/doc/current/security.html).

Answer 1

What's in the server and application log?
/var/log/nginx/...error.log
/path/to/web/app/logs/prod.log