Plesk interface is not working after Plesk upgrade: "TypeError: Cannot read property" or "404 Page Not Found"

Symptoms

• After updating (or upgrading) Plesk, several pages in the Plesk GUI fail to load, while showing one of the following error messages:
  

  PLESK_INFO: 404 Page Not Found

  ---

  PLESK_ERROR: TypeError: Cannot read property querySelector of null

  ---

  PLESK_ERROR: TypeError: Cannot read property 'map' of undefined.

  ---

  PLESK_ERROR: TypeError: Cannot destructure property 'isSecondaryCollapsible' of 'e' as it is undefined.

Cause

The Plesk update (or upgrade) process was interrupted and Plesk was upgraded partially, due to which it holds files from different Plesk versions, which are not compatible with each other.

Resolution

In order to resolve the issue, you must complete the Plesk update or upgrade process by following these steps:

1. Connect to the Plesk server via SSH (Linux) / RDP (Windows Server)

   Note: On Windows Server, you must also start a command prompt as an Administrator

2. Start the Plesk Installer by executing the following command in order to complete the upgrade:

   Note: The command is the same for both Windows and Linux servers

   plesk installer update