There's a major bug in Java 7 that prevents ALL transfers on Windows machines!
We've reported it at http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7077696
It's been accepted, but marked as "low priority". For anyone using FTP on Java/Windows it's certainly a very high priority.
PLEASE VOTE the bug up - here.
We're hoping to get it into the top 25.