`stretch-backports` kernel seems to have broken dependencies since a few days ago. In particular this package:
https://packages.debian.org/stretch-backports/linux-image-amd64 seems to have a broken dependency for linux-image-4.19.0-0.bpo.6-amd64, which does not seem to exist in the repositories (rather older linux-image-4.19.0-0.bpo.5-amd64 exists). `linux-headers-amd64` is fine and has correct dependencies.