Something is broken on the apt config on planet1001, apt-get upgrade would currently install systemd from backports, which is not what we want. planet2001 is fine, though:
jmm@planet1001:~$ sudo apt policy libpam-systemd libpam-systemd: Installed: 232-25+deb9u12 Candidate: 241-5~bpo9+1 Version table: 241-5~bpo9+1 100 100 http://mirrors.wikimedia.org/debian stretch-backports/main amd64 Packages *** 232-25+deb9u12 100 100 /var/lib/dpkg/status jmm@planet2001:~$ sudo apt policy libpam-systemd libpam-systemd: Installed: 232-25+deb9u12 Candidate: 232-25+deb9u12 Version table: 241-5~bpo9+1 100 100 http://mirrors.wikimedia.org/debian stretch-backports/main amd64 Packages *** 232-25+deb9u12 500 500 http://mirrors.wikimedia.org/debian stretch/main amd64 Packages 100 /var/lib/dpkg/status 232-25+deb9u11 500 500 http://security.debian.org/debian-security stretch/updates/main amd64 Packages