Myatu on "[Plugin: Background Manager] twig extension causing a 500 Internal Server Error (frontend/backend)"

With cPanel, the logs are usually located in /etc/httpd/domlogs/(domain)/. If you are using cPanel, you may have to enable it to log internal server errors first. This can be done by editing the /var/cpanel/templates/apache2/vhost.local file with:

ErrorLog [% serverroot %]/domlogs/[% vhost.log_servername %]-error.log

Additional logs can be found in the /var/log directory, ie., /var/log/apache2/error.log - these in particular would log information as to why 500 error was generated, rather than just reporting that it happened (as in a web server access log).