Let me guess, you're trying to use a 1.5.1 client or others' 1.5.3+ client to connect. Those won't work anymore.
You MUST use our full 1.5.3 client, found
here. Use the first DIRECT DOWNLOAD link to get the game.
DO NOT USE THE PWI-EPW patch, as that will corrupt most of your files (and make it a real pain in the butt to do any instances properly.)