There have been a lot of questions about jessie,
jessie-backports and jessie-updates.
Answers are here:
Summary from the end:
The jessie-backports suite was archived on archive.debian.org,
so you can use:
deb http://archive.debian.org/debian/ jessie-backports main contrib non-free
deb-src http://archive.debian.org/debian/ jessie-backports main contrib non-free
But then you will run into another issue:
E: Release file for
is expired (invalid since 36d 1h 9min 51s). Updates for this
repository will not be applied.
Unfortunately, with the APT version in jessie, this cannot be
ignored on a per source basis (it can with the APT version from
stretch, using the deb [check-valid-until=no] ... syntax). So
you need to disable this check globally, using:
echo 'Acquire::Check-Valid-Until no;' > /etc/apt/apt.conf.d/99no-check-valid-until
After that, apt-get update just works.