In my case, the problem happens because /etc/cron.daily/apt starts apt-key net-update, which ignores the apt proxy settings.
See https://bugs.launchpad.net/ubuntu/+source/apt/+bug/226780
In my case, the problem happens because /etc/cron.daily/apt starts apt-key net-update, which ignores the apt proxy settings.
See https:/ /bugs.launchpad .net/ubuntu/ +source/ apt/+bug/ 226780