While this error may be alarming, generally it will occur if a player is logged into the game when they initiate a character reset.
As mentioned in our Knowledge Base article going over the effects of a character reset, players will be removed from the game if they are logged in and locked out of their account until the reset has finished processing. Players should avoid logging in for at least one hour after initiating a reset to ensure everything has fully completed, even if they are able to login before that time has passed.
If the reset has not completed after several hours, attempt the reset again.