Flatpak installations error with 404

Hi * Trying to install programs from flatpak (flathub) on a fresh install of Zorin 16. It gets immediately stuck and finally faults out with 404. Isn't it supposed to function right out of the box? If so, perhaps something needs updating? Or do we need to do some configuration first?

Is it any program or a specific one?
If all programs are doing this, please run in terminal:

sudo apt update && sudo apt full-upgrade

Then test Flatpak. If still no good:

sudo apt install --reinstall flatpak