I was just wrapping up my night on Wyrmrest Accord Server, when I was disconnected from the game. Upon trying to log in, I received a message that the Login servers were full. Since there was only a small wait, I decided to see if it would reconnect. It immediately DCed me again with the error message: BLZ51901016 (Just tested it a second time to be sure it wasn’t a fluke).
I’m not sure if it’s a DDOS or just a case of Frontier being crappy, but I figured I’d pass it on in the event it was so Blizzard could resolve it before the morning rush of logins. Hope this info is helpful.