This has happened to me before, I try to play a particular knight and it will hang at the "Entering Ready Room" step. The client is not frozen, and communication to the server is not disrupted as far as I can tell. Trying to log off and log on to the same character results in getting stuck, but other characters on the same account are fine.
What seems to be happening is, the first time you log in there is some kind of server hiccup where you are considered logged in but disconnected, and it prevents you from accessing that character. The problem solves itself after ~10-15 min, which is about the amount of time it takes for an idle/disconnected character to be logged out. It's not a super-serious problem, but it is extremely annoying when it happens.