Various operations freeze and get stuck in Plesk: Request to ws-server failed

Symptoms

• Various operations (e.g. adding a domains / customer, importing a database) freeze and get stuck in Plesk.
• New task notifications (popup windows) do not appear in the bottom left corner in Plesk, unless the page is refreshed.
• When installing an extension from the Extensions catalog, the installation stays in the 'Queued' status:
  
  
   

• One of the following error messages is logged in /var/log/plesk/panel.log:

  CONFIG_TEXT: ERR [panel] Request to ws-server failed with response: 403 Forbidden

  CONFIG_TEXT: ERR [panel] The response callback is expected to resolve with an object implementing Psr\Http\Message\ResponseInterface, but rejected with "PleskObjectNotFoundException" instead.
  ERR [panel] Request to ws-server failed with response: 500 Internal Server Error

Cause

plesk-web-socket process is stuck.

Resolution