Hopefully "Fakename Q Notarealperson" won't get me arrested… Here's the password help message:

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Your password must be 8 to 30 characters in length and must contain at least one uppercase letter, one lowercase letter, and one number.

Your password is case-sensitive.

You can’t use personal identifiers such as your first or last name, date of birth, or Social Security number in your password.

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Here are some error codes the API returns:

  ["NULL_USERNAME", "NULL_EMAIL", "PWD_ILLEGAL_CHARACTERS", "PWD_CONTAINS_SPACE", "NULL_CHALLENGE_QAS"]

The UI doesn't expose the password error codes. It just says “You entered an invalid response. For more info, select the help (?) icon.” (The NULL_USERNAME and NULL_EMAIL errors seem to be spurious in this context.)