Thank you for trying to help!
I checked in /etc/libvirt/qemu and there are no files with -orig.xml. There is one file for each domain (guest OS), three in total. I am tempted to try removing them all.
I am restoring the configuration with sudo virsh define <domain>.xml before starting up the domains.
I will reload this host with Manjaro linux (rolling release to avoid the Ubuntu version upgrade every few years) in the near future as soon as I find time to rebuild this system. I hope the rebuild will totally eliminate this problem.