Downloading http://mirror.yandex.ru/debian/dists/stretch-backports/InRelease...

gpgv:  RSA key ID 2B90D010

gpgv:  RSA key ID B7D453EC

Downloading http://mirror.yandex.ru/debian/dists/stretch-backports/Release...
Downloading http://mirror.yandex.ru/debian/dists/stretch-backports/Release.gpg...

gpgv:  RSA key ID 2B90D010

gpgv:  RSA key ID B7D453EC

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