Stuck on finalize installation

I just re-install my Freedombox from scratch, and I’m stuck on the page “finalize installation” for more than 10 hours now.

Since I’m on an old raspberry, I installed Freedombox following this:

on a Raspbian. It worked well, I was able to create an admin account.

Then I try to move to backports by adding:
deb buster-backports main

apt-get -t buster-backports install -y freedombox

The only error I get was:
Errors were encountered while processing:
E: Sub-process /usr/bin/dpkg returned an error code (1)

Since, I’m stuck on “finalize installation”.

Does exim4 create the issue ?