Different Plesk interface operations are stuck and freeze frequently: Request to ws-server failed

Symptoms

• Various Plesk operations like adding customers/domains, importing databases freeze and stuck forever.

• Unable to install extensions at Extensions > Extension Catalog page, the installation is stuck on "Installing" step:
  

• The following errors can be found in /var/log/plesk/panel.log file:

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

  ---

  ERR [panel] The response callback is expected to resolve with an object implementing PsrHttpMessageResponseInterface, but rejected with "PleskObjectNotFoundException" instead.
  ERR [panel] Request to ws-server failed with response: 500 external Server Error

Cause

Stuck plesk-web-socket process.

Resolution