The site holds both IPv6 and IPv4 addresses
$ dig AAAA speedtest.tele2.net -> 2a00:800:1010::1
$ dig A speedtest.tele2.net -> 184.108.40.206
IPv6 is normally preferred if available. But while the server can be reached at the IPv4 address port 21 it cannot be given at the IPv6 address, i.e. an irregular setup of FTP and DNS.
The error message from Java is puzzling as it is not the IPv6 address which is bad but it fails to connect to this address.