Is this problem only with one specific file or you can’t transfer any files to vault?
Can you transfer files to any other VM except vault from work VM?
Can you transfer files between any other VM except vault and work VM?
This problem may be related to some failed updates.
You can check these files in dom0:
/var/log/dnf.log
/var/log/dnf.rpm.log
And search for err or fail strings and see if some packages failed to install.
Also did you try to update your dom0 and templates? Do you get any errors when you try to update?
Try installing a template that you don’t have (like debian-11-minimal, fedora-35, or any template that you haven’t installed yet), then make two AppVMs based on that new template and try to qvm-copy a file between them.