When using the Defender 5.7 Management Portal, if an account name is the same as the domain name, it is not possible to login to the Management Portal.
The message "Invalid domain/username or password" may be returned.
WORKAROUND: None. In Defender 5.7, it is not possible to login to the Management Portal with an account name that matches the domain name.
STATUS: This has been corrected in the Defender 5.8.1 Management Portal.
From the Defender 5.8.1 Release Notes; under the 'Defender Management Portal resolved issues' section: