Wow. Interesting timing. Yes, I just “solved” it this week. “Sovled” is in quotes because I’m not 100% certain it’s repeatable, so i’ll add steps that are probably irrelavant.
tried updaing dom0, got the error message
tried updating sys-whonix → the update of sys-whonix iteself failed
used sys-firewall instead of sys-whonix → still didnt work
used sys-net instead of sys-firewall → still didnt work
tried updating sys-net template → still didnt work
tried opening a terminal in sys-net and doing a sudo apt update; sudo apt upgrade. the live update of sys-net went fine
before restarting sys-net (which will undo the sys-net upgrade), i went back to dom0 and did the “sudo qubes-dom-update” command and this time it worked.
So I don’t have a good reason that it worked, but after doing that stuff it worked. Hope it helps.
Well good to know. If I ever have this problem again I’ll try your solution. I fixed it by re-installing and choosing Fedora as my default template instead of Debian and that fixed it. I had a new install anyways so a fresh re-install didn’t matter. Without reinstalling I bet you could just create a new sys-firewall and/or sys-net VMs based on Fedora and use those as your dom0 update VMs. Some problem with Debian not working with dom0 updates