Acho que seu problema pode estar relacionado a esse bug: link . Eu tentei usar o recurso de espelho no Ubuntu 14.04 também e deu alguns erros.
Você pode fazer o backup de seu arquivo /etc/apt/sources.list
sudo mv /etc/apt/sources.list /etc/apt/sources.list.orig
e crie um novo com o seguinte conteúdo. Substitua o ro.archive.ubuntu.com pelas iniciais do seu país (por exemplo, fr, de, us etc).
#############################################################
################### OFFICIAL UBUNTU REPOS ###################
#############################################################
###### Ubuntu Main Repos
deb http://ro.archive.ubuntu.com/ubuntu/ precise main restricted universe multiverse
###### Ubuntu Update Repos
deb http://ro.archive.ubuntu.com/ubuntu/ precise-security main restricted universe multiverse
deb http://ro.archive.ubuntu.com/ubuntu/ precise-updates main restricted universe multiverse
deb http://ro.archive.ubuntu.com/ubuntu/ precise-proposed main restricted universe multiverse
deb http://ro.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
###### Ubuntu Partner Repo
deb http://archive.canonical.com/ubuntu precise partner
###### Ubuntu Extras Repo
deb http://extras.ubuntu.com/ubuntu precise main
Em seguida, execute:
sudo apt-get update
sudo apt-get install openssh-server