Connect to FTP Server via FTP Proxy

Question

I try to poll files from a ftp server using ftp4j. My application runs behind a Squid Proxy, so I use HTTPTunnelProxyConnector to connect to the proxy. But when trying to connect, it throws the following exception:

java.net.SocketException: Can't connect to SOCKS proxy:Connection refused
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:397)
at java.net.Socket.connect(Socket.java:529)
at it.sauronsoftware.ftp4j.FTPConnector.tcpConnectForCommunicationChannel(FTPConnector.java:208)
at it.sauronsoftware.ftp4j.connectors.HTTPTunnelConnector.httpConnect(HTTPTunnelConnector.java:111)
at it.sauronsoftware.ftp4j.connectors.HTTPTunnelConnector.connectForCommunicationChannel(HTTPTunnelConnector.java:195)
at it.sauronsoftware.ftp4j.FTPClient.connect(FTPClient.java:1036)
at it.sauronsoftware.ftp4j.FTPClient.connect(FTPClient.java:1003)

This is the code snippet I use for the connection:

FTPClient client = new FTPClient();
client.setType(FTPClient.TYPE_BINARY);
client.setPassive(true);

HTTPTunnelConnector conn = new HTTPTunnelConnector("XX.XX.XX.XX", 80);
client.setConnector(conn);

try {
    client.connect("some.ftp.server.com");      // here I get the Exception

    ....

It seems that regardless of which kind of Connector I use, it always uses a SocksSocketImpl, which fails because there is no Socks Proxy. I already tried setting the proxy manually in Socket class, but get java.lang.IllegalArgumentException: Invalid Proxy for Proxy Type HTTP.

I can connect to the server with wget and the environment variable ftp_proxy set, so my problem seems to have nothing to do with proxy configuration, but I am no network crack, so I am not pretty sure...

I have no influence on the type of proxy installed, so I can not simply use another one. Any hint anyone please?