You attempt to use a Mobile Device Management (MDM) feature or enroll an Apple device. Despite this having worked in the past, these features suddenly stop functioning.
The Apple Push Notification (APN) certificate has expired.
Follow the steps in the article Apple Certificate Management under the renewal section to get a working APN certificate uploaded back to the account. Once this happens, devices will need to be re-enrolled in one of the following ways to regain MDM abilities:
Through the administration portal:
Manually on each device: