Various Plesk operations like adding customers/domains, importing databases freeze and stuck forever.
The following errors can be found in
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
Connect to the server via SSH.
# service plesk-web-socket restart