Stop giving duff info to other users. This isn't the first time. If you don't have an answer to the problem, or have no constructive instructions on how to fix the problem, or to assist with debugging, don't post an answer. Perhaps one day, someone will delete your account by mistake.
Back on-topic.
The error is produced when none of the conditions for registering match the criteria.
https://github.com/rathena/FluxCP/blob/5280fcdd11811e0a311cffafedc14f7bc01d6e88/modules/account/create.php#L152
If your error was based on the user's password containing the username, you would be presented with that error, not the one you have provided.
case Flux_RegisterError::PASSWORD_HAS_USERNAME:
$errorMessage = Flux::message ('PasswordHasUsername');
break;
The first place to look is in your error logs. Not all errors are displayed on the pages themselves, so you need to check in /data/logs/errors/exceptions/ and /data/logs/mysql/errors/.