One of the possible reasons of this problem is that many firewalls refuse connections from servers with enabled TCP ECN (Explicit Congestion Notification). To access a site behind such a firewall you will need to disable ECN on the hardware node by running
# sysctl -w net.ipv4.tcp_ecn=0
and adding the following line into /etc/sysctl.conf:
net.ipv4.tcp_ecn = 0
Disabling TCP ECN will not cause any problems.