Getting the same issue. Support emailed me saying,

"The error is most commonly caused by an improper disconnection from the servers during gameplay. As the game servers still recognize the character as being logged in due to the improper disconnection, the character cannot be logged into again. Unfortunately, there is no manual way to communicate to the servers that the character is no longer logged in, nor is there a way to manually log the character out. The character will eventually be recognized as logged out, though this time can vary depending on multiple factors, such as the area the character was in when the disconnection happened."

I read of people with this error having to wait until the next maintenance to log in as the game forces everyone to log out then, but I really don't wish to wait that long.