current stable image not able to upgrade

I gave the current amd64 stable image (from 2020-05-09) a spin, and it was not able to upgrade to the current backport enabled version.

Even repeated unattended-upgrades would not update anything (while still on an old version).

I only got it going with this manual workaround from https://discuss.freedombox.org/t/olimex-freedombox-will-not-update-from-19-2-to-latest/777/4 :

sudo /usr/share/plinth/actions/upgrades setup-repositories
sudo unattended-upgrade -d