This document explains the cause for the certification error encountered during backups which generally follows an SDDC maintenance or upgrade.
The Veeam backup server reports a certification error on connection to the cloud vCenter as shown below:
When the SDDC Upgrade and Maintenance activity is carried out, the ceritificates for vCenter are upgraded if they are about to expire. There is also a disconnection to the vCenter if the NSX edge appliance is being upgraded. The backup will only be impacted if a backup job is running and the vCenter goes down at the same time.
Avoid scheduling a backup job during the SDDC maintenance activity.