Adding some debug options might shine some light on it:
-o Debug::Acquire::http=1 -o Debug::pkgAcquire::Worker=1
(lots of output – "2>&1 | tee" advised)
W: Entrée « Date » non valable dans le fichier Release /var/lib/apt/lists/ftp.fr.debian.org_debian_dists_unstable_InRelease
W: The repository 'ftp://ftp.fr.debian.org/debian unstable InRelease' provides only weak security information.
[…]
W: Entrée « Date » non valable dans le fichier Release /var/lib/apt/lists/ftp.fr.debian.org_debian_dists_experimental_InRelease
W: The repository 'ftp://ftp.fr.debian.org/debian experimental InRelease' provides only weak security information.
Could be interesting to see the Date fields of those files as the ones
I see online are perfectly fine, but we had problems with parsing the
Date before thanks to a stdlibc++6 bug and I can't pretend that my
replacement is bugfree…
In unrelated news: I would strongly suggest to switch to http for the
debian sources.