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
Indeed it only happens with trusty at this point.
Updated test case:
sudo apt install squid3 proftpd-basic curl localhost: 3128/ curl --upload-file /tmp/zero ftp://ubuntu: ubuntu@ localhost/
echo -e "ubuntu\nubuntu" | sudo passwd ubuntu
touch /tmp/zero
ftp_proxy=http://
On affected versions, that will return an error from curl:
curl: (52) Empty reply from server