Good afternoon and thank you for posting in the Final Fantasy XIV Technical Support Forums.

This error is caused by the system still recognizing the character as logged into the instance. Usually it should take about 24 hours for the issue to resolve itself. Do you experience this issue with any other character? If it is only happening with this one character, you may want to try deleting the locally-saved data folder (My Documents > My Games > Final Fantasy XIV). Make sure to create a backup of this so you can replace it if it doesn't solve the issue.

Hope to see you online!