This error usually looks like this from syslog of the Proxmox node:

ERROR: job failed with err -1 - Operation not permitted 
INFO: aborting backup job 
INFO: stopping kvm after backup task 
ERROR: Backup of VM 101 failed - job failed with err -1 - Operation not permitted 

The primary cause of this issue is when the backup storage has less space than the total storage required for an assigned backup task. Verify the total storage space that is required for backing up the selected virtual machines.