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This is how I fixed it.
**Warning: ** You should perform a snapshot or a backup of your droplet before attempting to upgrade Ubuntu or modify the /etc/apt/sources.list
Ubuntu Utopic Unicorn reached end of life on July 25th 2015, so the apt-get server URL have changed from archive.ubuntu.com to old-releases.ubuntu.com. I’m not sure what is the problem with DO mirrors, I just end up replacing the whole content of /etc/apt/sources.list file with:
# /etc/apt/sources.list
deb http://old-releases.ubuntu.com/ubuntu/ utopic main restricted
deb http://old-releases.ubuntu.com/ubuntu utopic-security main restricted
deb http://old-releases.ubuntu.com/ubuntu/ utopic-updates main restricted
# Dépôts de sources (uniquement utiles pour télécharger les sources avec apt-get source. Dans ce cas, enlever les #)
# deb-src http://old-releases.ubuntu.com/ubuntu/ utopic main restricted
# deb-src http://old-releases.ubuntu.com/ubuntu/ utopic-updates main restricted
# deb-src http://old-releases.ubuntu.com/ubuntu utopic-security main restricted
deb http://old-releases.ubuntu.com/ubuntu/ utopic universe multiverse
deb http://old-releases.ubuntu.com/ubuntu utopic-security universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ utopic-updates universe multiverse
# Dépôts de sources (uniquement utiles pour télécharger les sources avec apt-get source. Dans ce cas, enlever les #)
# deb-src http://old-releases.ubuntu.com/ubuntu/ utopic universe multiverse
# deb-src http://old-releases.ubuntu.com/ubuntu/ utopic-updates universe multiverse
# deb-src http://old-releases.ubuntu.com/ubuntu utopic-security universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ utopic-backports main restricted universe multiverse
# Dépôts de sources (uniquement utiles pour télécharger les sources avec apt-get source. Dans ce cas, enlever les #)
# deb-src http://old-releases.ubuntu.com/ubuntu/ utopic-backports main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ utopic-proposed main restricted universe multiverse
# Dépôts de sources (uniquement utiles pour télécharger les sources avec apt-get source. Dans ce cas, enlever les #)
# deb-src http://old-releases.ubuntu.com/ubuntu/ utopic-proposed main restricted universe multiverse
## Commercial
deb http://archive.canonical.com/ubuntu utopic partner
## Sources
# deb-src http://archive.canonical.com/ubuntu utopic partner
To avoid that kind of problem in the future and continue to get security updates, I did an upgrade to a supported version of Ubuntu (ie: non end-of-life version) as explained here: http://askubuntu.com/questions/91815/how-to-install-software-or-upgrade-from-an-old-unsupported-release
Which could be resumed to:
sudo apt-get update
sudo apt-get dist-upgrade
# will open an additional SSH port (1022) in case of problem, you may have to disable firewall rules
iptables -I INPUT -p tcp --dport 1022 -j ACCEPT
sudo do-release-upgrade
Then follow the instructions. Be patient, the upgrade process will take a while to complete.
If everything go well, you will have Ubuntu Vivid Vervet (15.04) installed. Run do-release-upgrade again to update your droplet to the latest supported version of Ubuntu (15.10 aka “Wily Werewolf”).
The next Ubuntu Long Term Support (LTS) release should be out next month, so, I plan another upgrade soon.
Hope it helps!