One of the answers in Debian: updating package lists with apt-offline, --simulate not understood for apt-get update mentions that the issue was fixed in Ubuntu 14.04.
However, on my Ubuntu 14.04 system, the command
sudo apt-offline set apt-offline.sig still fails with the same error messages. Does anyone know whether
apt-offline will be fixed soon in Ubuntu 14.04, so it can, for example, be used to update offline machines to the upcoming 14.04.1 release?