Aside from it being a possible bug, two things that might help you resolve it.

The first, if you are on PC, is to go to Documents/My Games and go to the ffxiv folder, backup the CHAR folders by dragging them to the desktop, then make sure they are not in the ffxiv folder anymore. Then login to the game. It resets all your settings in case some of them got corrupted and crash you. If it works, you can slowly drag some of the setting files back one at a time until the crashes start, then you know which one is the cause.

The second is to click visit another world server on the character selection screen. It will take you to Ul'dah on a random server.