I certainly may not have run the qubesctl command correctly, but the gui based update utility also fails. I had changed the default disposable template to fedora-34-qubes-core. Does it need to be a centos-8 template to work?
[root@dom0 ~]# qubesctl --skip-dom0 --targets=centos-8 --show-output --force-color state.sls update.qubes-vm
(Attachment updating-centos-8-fails.txt is missing)