Today I had to start to basically login twice WHY? If I’m login to the interface why do I have to login into the game too? I have two accounts and I have never had to login like this before. Plus been here from day 1 too.
While multiboxing (playing more than one account) is allowed, it is not officially supported.
If you are using the launcher make sure you have it set to allow multiple instances of the launcher. The option is under Settings > General
If that is not it, then hopefully another player who does multibox can assist you.
I had that happen to me today too. I just exited the game and reloaded it from the launcher and it worked fine.
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I tried that and it didn’t work. But I did try it again and this time it work.
Been logging in since 2012 with no problems. Been logging in since 2017 on the 2nd account with no problem. When I which accounts all I do is log out of one and login on the other on the interface and right into the game.
But since I restarted everything it seam to be fine now. Plus with out the box check too.
Well, whatever caused it I am glad you are able to play now without issues! 
I only have one account and this happens to me sometimes. Doing this always does the trick though.