This happens whenever attempting to log in. I did log in once today just after servers went live. I logged out and since then, attempting to "Start [the] Game" has been throwing errors. The process being:
Load Game
Agree to Terms
Select Character (character creation is available)
Click "Start Game"
Moments later (while the queue ticks down) a popup with the message "Disconnected from server.(300)" appears
Client closes shortly after
I was able to create a character as long as I didn't click the "Start Game" button after the process was complete.
I attempted to create a character on another server. This had no effect.
I decided to turn off Norton's Smart Firewall for the time being. The queue numbers have changed as well from when I was getting the errors. On this run attempt, the error has yet to appear (I'm quite surprised. This has never had an effect before). I will update with more information.
Picture Below:
http://imgur.com/uDjIBLp