This error seems to come from QEMU and the SATA driver: Looks like this driver wants write-access to the source-image, but Proxmox define them as read-only when backing up templates.
I'm not going to dig into QEMU-Source so this is my workaround:
Hi,
when I try to backup a vm-template with a SATA-Disk it fails:
It works when I change the Disk type to SCSI:
It also works when I use SATA and remove the "tempalte" marking:
Is this by design or a bug?