Plesk admin receives the following notification via email:
Unable to update EXT.XXXXXXXX.XXXX: an error occurred while processing your key. You can try to retrieve the key manually in Plesk (Tools & Settings > License Management) or let Plesk try to update it in the next 24 hours. If you do not want to receive this notification any more, remove this key from Plesk.
The license key is expired. Some features can be unavailable. Please contact your hosting provider.
Last four digits of the additional key represent a large number e.g.
Previously installed trial Plesk license key was bundled with an additional key that has expired.
Go to Tools & Settings > License Management > Additional License Keys
Select the key
EXT.XXXXXXXX.1234that is mentioned in the notification