Have you looked at the connection logs (specifically looking at the token sent to the server), and checked that the token being sent is a valid token? It could be that somewhere a space has gotten added during copy/pasting, or some error on your clients end and how they’re sending the token.
Can any of the 13 others who can connect without issue try to connect using the token from the person with issues? If it works for them it shows it’s not a token issue, and the issue lies with the user failing to connect and something going on at their end.