Comment 17 for bug 1423498

Indeed it only happens with trusty at this point.

Updated test case:

sudo apt install squid3 proftpd-basic curl
echo -e "ubuntu\nubuntu" | sudo passwd ubuntu
touch /tmp/zero
ftp_proxy=http://localhost:3128/ curl --upload-file /tmp/zero ftp://ubuntu:ubuntu@localhost/

On affected versions, that will return an error from curl:
curl: (52) Empty reply from server