My tests indicate the site has a broken firewall, tripping over the TCP window scaling option. You can get around this by tuning down the max parameter (the third parameter) in /proc/sys/net/ipv4/tcp_rmem, but I would recommend you contact the owner of the site and inform them about the problem.
Hendrik, are you using a tool which is available in the public domain ? If so, which one ? Thx for all info. Marc.