Which of the following is NOT a reason for failing to log in?

The correct choice identifies a scenario that does not contribute to a failure to log in. Entering a valid password suggests that the credential provided is accepted by the system, indicating that the user is inputting a recognized and correct value for password authentication.

In contrast, incorrect username, expired password, and system updates can all impede access. An incorrect username may lead to a login failure because the system cannot find a user associated with that name. An expired password would result in access denial because the credentials must be current for authentication. Similarly, if the system is undergoing an update, it may prevent logins altogether, as certain functionalities might be temporarily unavailable.

Thus, the presence of a valid password shows that login is achievable when combined with the correct username and unaffected by external factors, reinforcing that this scenario does not contribute to login failure.