One by one.
ipv6 is the next internet protocol that can handle longer page-ids/ips (numbers and letters) than the current ipv4 protocol. ipv6 is known to cause problems with many routers as those are not yet compatible with ipv6 but only know the established ipv4 protocol, thus transfers will get blocked at times.
The easiest fix for your problem is (assuming you use an ext3 partition format) to find the IP address of your Internet Service Providers DNS servers e.g. with google, or a phonecall or to use a free DNS server IP and put that into /etc/resolv.conf. For this you will need a basic editor like pico, nano or vi.
open the file with e.g. nano /etc/resolv.conf
change the namerserver bla.bla.bla entry and save the file. Now exit the editor and type
chattr +i /etc/resolv.conf
this will hard-lock the file until you remove the lock with chattr -i /etc/resolv.conf. Now, you should be able to connect to ftp-servers again.

ipv6 can also be disabled in a config file, I think it was something like /etc/modprobe.d/aliases (long time ago since I used debian...) and change the following entry
alias net-pf-10 ipv6 to alias net-pf-10 off
and add also
alias net-ipv6 off
save the file and exit.

Hope it helps.