Have you selected a map and a game mode when hosting? If that didn't work, try setting the map manually after you start a server by opening the console with ~ and then typing in "map Towers" or "map Arena"
Are you using the updater or the zip file? You can also try updating to the latest version, and then running the "reset" command in the console. This should delete all the existing configs for the game and restart the game from scratch.
