ERROR: Backup of VM 450 failed - unable to open '/mnt/pve/nas/dump/vzdump-qemu-450-2022_11_22-15_51_26.tmp/qemu-server.conf' at /usr/share/perl5/PVE/VZDump/QemuServer.pm line 210
Hi,
if it's a CIFS share it might be the same issue from this thread (in German). The solution/workaround was to mount the CIFS share with an fstab entry with the noserverino option and then re-add that as a Proxmox VE directory storage with is_mountpoint 1.