# Wrong URLs or uncomplete .deb packages when retrieving them from an FTP?
# OK, drop (and rename) this file into /etc/apt/apt-conf.d/70passiveftp 
# (create apt-conf.d/ if not present),
# then just run apt-get update, and wait ...
# This will probably fix your problem, turning passive ftp mode on.

Acquire
{
  ftp {
    Passive "true";
  };
};