By now upgrading seems impossible. The distro has reached "end of life". However the next release is not available. Many users have the same issue. I tried:
740 apt update
741 apt full-upgrade -y
742 apt search update-manager-core
743 do-release-i
744 do-release-upgrade
745 do-release-upgrade -d
746 apt full-upgrade -y
747 apt install -y update-manager-core
748 apt clean
749 apt update
750 do-release-upgrade
751 apt update
752 apt full-upgrade -y
753 apt clean
754 apt update
755 reboot
And the GUI "Software Update" did not work either. I'll just wait a week or two and see.
Since a lot of users have the same issue I've made a formal bug report for Ubuntu.
