Proxmox backups failing for a single VM
I use Proxmox Backup Server for my Proxmox backups.
Every VM on my Proxmox node(s) was backing up fine except one, which failed every single night with the following error:
INFO: starting new backup job: vzdump 100 --node pve03 --notes-template '{{guestname}}' --notification-mode notification-system --storage node-backup01 --mode snapshot --remove 0
INFO: Starting Backup of VM 100 (qemu)
INFO: Backup started at 2026-04-28 01:33:24
INFO: status = running
INFO: VM Name: pihole01
INFO: include disk 'scsi0' 'local-lvm:vm-100-disk-0' 10G
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating Proxmox Backup Server archive 'vm/100/2026-04-28T00:33:24Z'
INFO: issuing guest-agent 'fs-freeze' command
INFO: starting backup via QMP command
INFO: issuing guest-agent 'fs-thaw' command
ERROR: VM 100 qmp command 'backup' failed - backup connect failed: command error: client error (Connect)
INFO: aborting backup job
INFO: resuming VM again
ERROR: Backup of VM 100 failed - VM 100 qmp command 'backup' failed - backup connect failed: command error: client error (Connect)
INFO: Failed at 2026-04-28 01:34:04
INFO: Backup job finished with errors
INFO: notified via target `ntfy`
Look at the VM name: pihole01.
Is the failing VM your internal DNS server? If so, you probably have the same problem!
Here's what happens: Proxmox issues fs-freeze on the guest before taking the snapshot. Your Pi-hole VM is now frozen, which means your DNS server is now frozen. Proxmox then tries to connect to the Proxmox Backup Server by hostname, that lookup goes to the DNS server it just froze, and the connection fails. Backup aborts, VM thaws, DNS comes back, and everything looks perfectly healthy by the time you go investigating.
The fix is to make the Proxmox host resolve its backup server without needing DNS at all. On the Proxmox node that has the VM (or all the nodes, if you have HA or move the VM around):
vim /etc/hosts
Add an entry for your backup server:
192.168.1.10 proxmox-backup-server.diogoferreira.pt
That's it. No restart needed, the next backup run will just work.