Login mask, User field

How can User field of the login view claim one can input email or username? The latter one does not work, user double checked her name input for possible errors. What immediately works is email address as user name. Too fool herself can user by itself, thirds are not needed.

It works for me using my name: diddledani. Could you paste their username here so that the admins can investigate (I’m not an admin)?

Apparently an error in test. More investigation however needed to understand problem nature.