The reason for the verification error is some missing or broken file. The backups on server A are done to a SAN and the SAN seems to have a problem from time to time. My guess is, that it becomes unavailable during backups and then the backup...
I have a two server setup PBS where server A is located within a proxmox cluster and recieves the backups for the proxmox VMs.
Server B is geographically separated and has a sync job to the datastore on server A. (the backups on A are...