Unable to access a website or access Plesk System Updates: Error 500 MySQL server has gone away

Symptoms

• Unable to access the Drupal website with the error:

  500 error

  ---

  Additional uncaught exception thrown while handling exception

• Some database operations via WordPress plugin may fail (for example, backup/restore with plugin WPVIVID)

• Going to Tools & Settings > System Updates, the Plesk System Updates page might fail to be accessed with the following error:

  Server Error
  500 PDOException
  SQLSTATE[HY000]: General error: 2006 MySQL server has gone away

Cause

Insufficient value for max_allowed_packet or wait_timeout parameters.

Resolution

Increase  max_allowed_packet or wait_timeout parameters according to the article How to set the value of max_allowed_packet or wait_timeout for MySQL on Plesk server?