When clicking Upload for the CSR file, there is no APNS certificate available for download.
Instead, the portal redirects to https://www.apple.com/filenotfound
MDM Push Certificates are critical for the operation of managed devices, if they expire, all devices will have to be reenrolled creating a catastrophic event for all the customers devices.
Please review and given how critical this service for renewing certificates is for your customers, please also make sure it is always available without downtimes.
Let me know if you need more details, Thank you, Sergio