Downloading http://archive.debian.org/debian-archive/debian/dists/stretch-backports/InRelease...
Success downloading http://archive.debian.org/debian-archive/debian/dists/stretch-backports/InRelease

gpgv:  RSA key ID 22F3D138

gpgv:  RSA key ID 386FA1D9

Downloading http://archive.debian.org/debian-archive/debian/dists/stretch-backports/Release...
Success downloading http://archive.debian.org/debian-archive/debian/dists/stretch-backports/Release
Downloading http://archive.debian.org/debian-archive/debian/dists/stretch-backports/Release.gpg...
Success downloading http://archive.debian.org/debian-archive/debian/dists/stretch-backports/Release.gpg

gpgv:  RSA key ID 22F3D138

gpgv:  RSA key ID 386FA1D9

ERROR: unable to fetch mirror: verification of detached signature failed: exit status 2