the fix is already available up to pve-enterprise since last week (in qemu-server 6.2-14). if you still have problems backing up VM templates, please post the full config, pveversion -v output and logs!
Here the log:
INFO: starting new backup job: vzdump 144 --mode snapshot --node hal9030 --storage pvebackup --remove 0
INFO: Starting Backup of VM 144 (qemu)
INFO: Backup started at 2020-09-07 09:26:06
INFO: status = stopped
INFO: backup mode: stop
INFO: ionice priority: 7
INFO: VM Name: Win7x64
INFO: include disk 'ide0' 'vmdata1zfs:base-144-disk-0' 48G
INFO: creating Proxmox Backup Server archive 'vm/144/2020-09-07T07:26:06Z'
INFO: starting kvm to execute backup task
kvm: -device ide-hd,bus=ide.0,unit=0,drive=drive-ide0,id=ide0,bootindex=100: Block node is read-only
ERROR: Backup of VM 144 failed - start failed: QEMU exited with code 1
INFO: Failed at 2020-09-07 09:26:07
INFO: Backup job finished with errors
TASK ERROR: job errors
pveversion:
proxmox-ve: 6.2-1 (running kernel: 5.4.44-2-pve)
pve-manager: 6.2-11 (running version: 6.2-11/22fb4983)
pve-kernel-5.4: 6.2-6
pve-kernel-helper: 6.2-6
pve-kernel-5.3: 6.1-6
pve-kernel-5.4.60-1-pve: 5.4.60-1
pve-kernel-5.4.55-1-pve: 5.4.55-1
pve-kernel-5.4.44-2-pve: 5.4.44-2
pve-kernel-5.4.41-1-pve: 5.4.41-1
pve-kernel-5.3.18-3-pve: 5.3.18-3
pve-kernel-4.13.13-5-pve: 4.13.13-38
pve-kernel-4.13.13-4-pve: 4.13.13-35
pve-kernel-4.13.13-3-pve: 4.13.13-34
pve-kernel-4.13.13-2-pve: 4.13.13-33
pve-kernel-4.13.13-1-pve: 4.13.13-31
pve-kernel-4.13.8-3-pve: 4.13.8-30
pve-kernel-4.13.8-2-pve: 4.13.8-28
pve-kernel-4.13.8-1-pve: 4.13.8-27
pve-kernel-4.13.4-1-pve: 4.13.4-26
pve-kernel-4.10.17-4-pve: 4.10.17-24
pve-kernel-4.10.17-2-pve: 4.10.17-20
ceph-fuse: 12.2.11+dfsg1-2.1+b1
corosync: 3.0.4-pve1
criu: 3.11-3
glusterfs-client: 5.5-3
ifupdown: residual config
ifupdown2: 3.0.0-1+pve2
ksm-control-daemon: 1.3-1
libjs-extjs: 6.0.1-10
libknet1: 1.16-pve1
libproxmox-acme-perl: 1.0.4
libpve-access-control: 6.1-2
libpve-apiclient-perl: 3.0-3
libpve-common-perl: 6.2-1
libpve-guest-common-perl: 3.1-3
libpve-http-server-perl: 3.0-6
libpve-storage-perl: 6.2-6
libqb0: 1.0.5-1
libspice-server1: 0.14.2-4~pve6+1
lvm2: 2.03.02-pve4
lxc-pve: 4.0.3-1
lxcfs: 4.0.3-pve3
novnc-pve: 1.1.0-1
proxmox-mini-journalreader: 1.1-1
proxmox-widget-toolkit: 2.2-10
pve-cluster: 6.1-8
pve-container: 3.1-13
pve-docs: 6.2-5
pve-edk2-firmware: 2.20200531-1
pve-firewall: 4.1-2
pve-firmware: 3.1-3
pve-ha-manager: 3.1-1
pve-i18n: 2.1-3
pve-qemu-kvm: 5.0.0-13
pve-xtermjs: 4.7.0-2
pve-zsync: 2.0-3
qemu-server: 6.2-14
smartmontools: 7.1-pve2
spiceterm: 3.1-1
vncterm: 1.6-2
zfsutils-linux: 0.8.4-pve1