i think i found out why this is happening...
Since I am using the stunnel, HFS sees the connection all from same IP, which is 127.0.0.1, since it considered the download from same origin, thus only allow 1 download of the same file at the same time. Even though the connection actually originated from different IPs.
I checked this by using the using h t t p : / / hostname:8080 on different computers and download the same file, I was able to do so... however if I open the same address from the same computer using 2 browsers (IE and firefox), then i can only download 1 at a time, the other browser tells me max download reached.
I got around this by turning off "prevent leeching (download accelerator)"
Thanks for the help, this is a great software.