It's not server congestion, it's a problem with the connection between the open world and the instance & housing servers. The connection may be physical or software, as SE don't give out that information we don't know. They may even be in the same physical system (computer if you will) but simply aren't talking to each other.

To explain it in a simple way:

1)When you input your login details you connect with the LOGIN server;
2)LOGIN server allows you to select DATA CENTRE server & connects you to that;
3)DATA CENTRE pulls up your characters on that set of worlds. You choose one and connect to the WORLD (Leviathan in our case), INSTANCE (dungeons etc) or HOUSING servers;
4)When you move from INSTANCE or HOUSING into the WORLD a connection is formed and vice versa.

In the situation we're having every server is allowing us to connect except WORLD, where the open world of Leviathan is (cities, areas, mobs). It's an inter-server issue. We can connect to all other WORLD servers except Leviathan. Only that server is having an issue.

NB: I use the term server but it could also be a service depending on if it's a hardware or software solution.