What does it mean?
This error is caused by some automatic "potential hacker" detection software that is run on the authentication servers. To wit: "Our authentication server detected a change in your IP address, and denied you entry because of a potential 'man in the middle' hacking attempt."
Why does this happen?
When you try to connect to the game, it connects first to the authentication server, and then the game servers. If the system detects that connection coming from 2 different IP addresses, it flags you as a potential hacker.
What should I do?
We've seen this problem happen on occasion to people using Wi-Fi at their school, or routing through a proxy server. Our best solution at this point is to go through a wired connection, or find an access point with less traffic.
Unfortunately this isn't something that we can disable, since we don't want hackers to be able to log in to other people's accounts and steal characters or items that don't belong to them. This should be a rare issue, but if it does happen to you, try to simplify your internet connection as much as possible. Try a direct, wired connection, and make sure you're not connecting through a proxy server.