After apt-get update, attempting to install ntp tries to download version 1:4.2.8p10+dfsg-3+deb9u1 and fails. It tries to download +deb9u1 because
$ aptitude show ntp Package: ntp Version: 1:4.2.8p10+dfsg-3+deb9u1 State: not installed [...] and it fails because the version of the package in the Debian 9 mirror listed in /etc/apt/sources.list is +deb9u2 : ntp_4.2.8p10+dfsg-3+deb9u2_amd64.deb I don't understand what went wrong. apt-get update seemed to go well, only complaining about missing "DEP-11 64x64 Icons", which are presumably not a vital part of ntp. How is this possible ? I'm confused.