Change the backport-entry too "o=Debian Backports,n=jessie-backports,l=Debian Backports"

With unattended-upgrades -v -d you can see which sources are "allowed" for unattended-upgrades  .

Change the backports entry to

"o=Debian Backports,n=jessie-backports,l=Debian Backports";

BTW, with

unattended-upgrades -v -d

you can see which sources are "allowed" for unattended-upgrades.