Account is already in use can be caused by a few things:
1. The most likely occurence. The game is lagging a bit behind the scenes, and it hasn’t registered that you’ve left one area yet, so it doesn’t want you to appear in two places at once.
2. You’ve logged in on one place, and are trying to log in on the same account elsewhere.
3. Someone else is logged into your account (least likely, if you’re careful)