Comment 10 for bug 985441

It would appear that this problem has occurred before (see http://www.webupd8.org/2010/04/medibuntu-repository-down-what-to-do.html. This site suggests a number of alternative sites for medibuntu, and a fix for the Medibuntu offline error.

I quote
Update: there's an even easier way to fix this (thanks to defrysk). Apparently only the DNS are having issues so you can fix Medibuntu by keeping original address and pasting this in a terminal:
echo "88.191.101.8 packages.medibuntu.org" | sudo tee -a /etc/hosts

This puts the Medibuntu IP address in the hosts file.

I restarted the system, after running the command in a terminal, and I am now able to access the Medibuntu reepository.

So problem appears to be associated with DNS.