NX gives you “session test failed” and “Failure negotiating the session in stage ‘7’”?

… then check your iptables settings on the server. Perhaps you made the same mistake as I did: Restricting local traffic (on the ‘lo’ interface).

Add the following rules to your iptables ruleset, and make sure it’s at the beginning (before the restrictions):

iptables -A INPUT -i lo -j ACCEPT