Troubleshooting "500 Internal Server Error" Drupal

1. If possible run cron and clear caches (1-9) with Drush
2. Change of PHP version to match the Drupal version used. For older Drupal 7 versions possibly downgrade to PHP 5.6. For Drupal 8 possibly upgrade to PHP 7.0 +
3. Increase PHP Memory limit to at least 256M
4. Copying of original .htaccess and settings.php from Drupal installation. Disable/Remove to see if it is the cause of the problem.
5. Check-up and correction of user rights, 755 and 440 special files
6. Run update.php (set TRUE for free acess in /sites/default/settings.php, go to www.domain.xxx/update.php and follow the instructions)

See also:
https://www.drupal.org/node/416906
https://www.drupal.org/node/2764539