On "Problems" and in "Last 20 issues" widget on the main page, the "Actions" column will show red "Failures" status indefinitely if at least one alert notification has ever failed (for example, due to "no media defined" for a user).

There is no way to reset/clean this status (other than remove the user and re-create it again). New successful alert notifications (after a media is defined, for example) won't clear up this status.