I have found 2.6-test10.1.95, at least, has ECN turned on. This was
breaking many websites for me. Many firewalls are known to have issues
with ECN. One example is at least some PIX firewalls. Sadly it seems to
more corporate the site, the more likely it is to break.
Workarounds:
echo 0 > /proc/sys/net/ipv4/tcp_ecn
net.ipv4.tcp_ecn = 0 in /etc/sysctl.conf
Fix:
Recompile kernel without ECN support
P.S. arjanv Please Fix!