You most likely are not hacked, but I'm assuming that you were playing, got DCd or booted for some reason, tried to log back in, and that's when you got the message.
The "bug" as you call it can happen at any time in that scenario, and I don't think "bug" is the right way to describe it; it's more of a connection issue. That may not sound right, either, since it can happen after being booted, but being booted is generally caused by a connection issue-- not a lack of connection, more the lack of a steady connection, where messages aren't flowing steadily and get held up, then are sent in a clump, which sometimes causes the server to boot you. At least, that's what I've gathered from reading about it when it used to happen to me more regularly. It isn't necessarily a problem with your internet connection per se; it might be that there's some other process running in the background on your computer that's hogging the connection, or some other device in your household might be using the internet, too.
When it happens, it's usually just a matter of waiting a few minutes for the server to actually log out your account so you don't get the message. I used to close the program and reboot, to clear my memory, and that's usually long enough for the account to be logged out so you can log in again.
Now that I play on both NA and EU servers, I usually just switch servers for a bit, then switch back.
As Maxx7410 has said just restart the game. Happens to me to sometimes and just restarting the game fixes the problem. It did give me a heart attack the first time cause I thought to myself "Did my account just get broken into?" Then I remembered I had 2-step Authentication and didn't see anything out of the ordinary.