If you have no errors in your logs, then nothing is throwing one, which means that a function somewhere somehow stucks your character, and believes it has done a correct job. Do you have any idle checks that are supposed to automatically dc people? Or any scheduled Task that has a deadline set for some hours after being created?
Note: I'm assuming that by "character stuck" you mean that the person can't login, and instead receives a message like "This ID is already logged in" when they try to, even after they have restarted. Is this correct?
If yes, then it looks like something is disconnecting the player without actually updating the database to reflect the disconnection, meaning that the player isn't connected, but the database believes it is, and, since a restart wouldn't influence this variable, for players that are supposed to be already offline server-side, then this carries even after the restart.
What you could do is check where, in the database, there's the variable that tells if an account is connected or not. It's most likely to be in a table such as "Accounts", something like, uh "loggedin", "login_state", or similar. This column should be changed whenever a player's logged in state changes, but apparently some function in your code doesn't perform a correct dc, if left running for some hours.
This is just a wild guess, by the way.
