- Are you running the backup through an AppVM? Doing backup in dom0 may cause bugs or be really slow. I believe this is involved in the python/admin code in Qubes 4, and is therefore different than it was in Qubes 3.2. So in Qubes 4, be sure to do it in an AppVM, freshly made in Qubes 4. The general idea I believe is to allow to network the backup process, that's why it was made to work in an AppVM. No attention was given to allow backup in dom0 here, probably on purpose since nothing should be done in dom0 anyway.
- Is the template/AppVM you're using specifically to run the backup based on a Qubes 4 template/AppVM? While Qubes 3.2. AppVM's can work in Qubes 4, it's generally my experience that they are more buggy/slow, and work better if re-made in Qubes 4 and then transfer the files over. Personally I took no chances and purged all my Qubes 3.2. templates/AppVM's in favour for fresh Qubes 4 ones. Removing Qubes 3.2. templates is obvious due to likely code differences in the qubes-core-agent-* packages, etc., but the AppVM is more controversial without confirmation and based on anecdotal experience. Generally I experienced improvements by purging Qubes 3.2. AppVM's in favour for freshly Qubes 4 made ones.
- Did you update dom0? I've also seen the qvm-create not working, but this only happens on a buggy install of Qubes 4, typically with python errors on the last step during first boot when creating default VM-configuarations and networking VM's. Generally I solved it either by re-install with different UEFI/BIOS settings, EFI/Grub settings, UEFI/BIOS update, try switch between EFI/Grub boot methods, loading different drivers.
Generally the most effective method to get Qubes 4 RC-1/2/3 to work on a system that I had issues with, was to unplug the drive, put it in a machine that works with Qubes. Then install Qubes, update it fully, and then put it back into the first machine again. Usually always work for me on any machine that on paper should support Qubes. Also Grub is easier to do this, as EFI paths can be a pain to adjust, they change when moving from a machine to another, while Grub does not change. Also be careful if you install on another machine, might be a good idea to remove the other drives first, just in case. Also EFI installs on a machine with existing EFI paths may cause issues.
All in all in short, use Grub is recommended here, and also to unplug other drives on the install machine you use.
Assuming you did not update dom0 due to the above bug, then once you get it updated, everything "should" work much, much better.
Np's :) I hope you succeed!
I've sometimes had longer issues with the UEFI/BIOS settings. I remember once when I started using Qubes, I used a couple of days to figure out why I could't install Qubes on a z170 Asus motherboard, i5 6500 CPU. Then it turned out it was a UEFI/BIOS setting to allow two graphic cards (one intel onboard, the other extenal nvidia card). Quite frankly, I was stunned by the cause, sometimes it's settings we least expect to cause any problems. The mix of VT-d, hypervisors and other exotic things in Qubes, seems to make it more sensitive to some UEFI/BIOS settings.