[Möph]

(If you haven't set up Flathub before, run `flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo` first.)

Actually, this point in brackets was the solution.

For me as a non-flatpak-user its kind of strange to "hey just refence here a random url" but it seems to work.
I could finally install the flatpak.

Thanks for your help.