When you visit the ListenBrainz page without logging in, then follow the navigation sequence "About" → "Submitting data", which opens https://listenbrainz.org/add-data/, clicking on the "Missing data" link brings you to a 404 error page with the URL https://listenbrainz.org/user//missing-data/

It looks like the username was intended to be substituted into this URL, but that's not possible when the user is not logged in.