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

Symptoms

• Plesk is not accessible with one of the following error messages in a web-browser:

  PLESK_INFO: Server Error 500
  
  Call to undefined method Plesk_Mode::isExternalOwner()
  Type Error
  Message Call to undefined method Plesk_Mode::isExternalOwner()
  File login_up.php

  ---

  PLESK_INFO: ERROR: Server Error 500 DI\
  NotFoundException
  No entry or class found for 'Plesk\GraphQl\Service'
  Type DI\ NotFoundException
  Message No entry or class found for 'Plesk\GraphQl\Service' File Container.php Line 135

  ---

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

  ---

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

  ---

  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: 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: Warning: Unknown: failed to open stream: No such file or directory in Unknown on line 0
  Fatal error: Unknown: Failed opening required 'auth.php' (include_path='.:') in Unknown on line 0

  ---

  PLESK_INFO: PHP Fatal error: During class fetch: Uncaught ParseError: Unclosed '(' in /usr/local/psa/admin/plib/Os/UtilTrait.php:104

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

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

• When creating a new subscription in Plesk at Subscriptions > Add Subscription or a new database at Domains > example.com > Databases > Add Database, the operation fails with:

  PLESK_ERROR: Zend_Form::isValid expects an array

Cause

The plesk-core package is corrupted.

Resolution