Error 500 is displayed on a website in cases where the server cannot process the site’s PHP code or .htaccess rules due to errors found in them. By default, errors are written to the logs/domain.com.error.log file. You can view them at panels management, in the WWW -> Logs section or in the file itself via FTP. If you want to enable the display of errors on site pages, create a .htaccess file in the site folder or open it if it exists and write the following lines:

php_flag display_errors on 
php_value error_reporting 7