If you are running Drupal 8 with a multisite setup or using Acquia Site Factory, it is recommended to use a unique hash_salt for each site. This unique hash_salt is sometimes referenced by backend processes and if not defined could lead to different types of plugin errors that are difficult to track down. Please read below to find out how to check if you are using unique hash_salts.
For each site, run this Drush command and replace "example.com" with your site name. Use Drush aliases to run Drush remotely.
drush --uri=example.com php-eval 'echo \Drupal\Core\Site\Settings::getHashSalt();'