Plesk admin receives notifications below by email:
Unable to update EXT.01234567.0000: 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.
The license key from the error message is an additional license key for, for example, the KernelCare extension. The subscription for this extension has been canceled recently.
The expiring additional license key
EXT.01234567.0000 is still attached to the main Plesk license.
Unlink the additional license key
EXT.01234567.0000from the main Plesk license key:
Navigate to Tools & Settings > License Information > Additional License Keys tab.
Find the key from the email - in this example it is
EXT.01234567.0000. Select it and click on the Remove button.
Switch back to the Plesk License Key tab and click on the Retrieve Keys button.