Plesk interface is not accessible: Class 'path\to\class' not found

Symptoms

• Plesk is not accessible with one of the following error messages:

  PLESK_INFO: Error: Class 'Plesk\EmailActivation\EventListener' not found

  ---

  PLESK_INFO: ERROR: Zend_Exception: File "Zend/Db/Statement/Pdo.php" does not exist or class "Zend_Db_Statement_Pdo" was not found in the file

  ---

  PLESK_INFO: ERROR: Class 'Composer\\Autoload\\ClassLoader' not found (autoload_real.php:23)

  ---

  PLESK_INFO: ERROR: require_once(): Failed opening required 'vendor/autoload.php' (include_path='/usr/local/sb/include:/opt/psa/admin/plib:/opt/psa/admin/externals') (init.php:4)

  ---

  PLESK_INFO: PHP Warning: Unknown: failed to open stream: No such file or directory in Unknown on line 0
  PHP Fatal error: Unknown: Failed opening required 'auth.php' (include_path='/opt/psa/admin/plib:/opt/psa/admin/externals') in Unknown on line 0

• When accessing the Domains menu, the following error message is shown in Plesk:

  PLESK_ERROR: Call to undefined method Plesk\Session\Context::getUser

• Unable to add a new subscription at Subscriptions > Add Subscription or a new database at  Domains > example.com > Databases > Add Database, the error below occurs:

  PLESK_ERROR: Zend_Form::isValid expects an array

Cause

The plesk-core package is corrupted.

Resolution