receiving error 401.1 with FQDN Authentication. Why?

[ThunderMusic]

Hi,
We are using FQDN Authentication on our web site. The site should
receive "DOMAIN/Username" for each user that logs on. The thing is, we
receive a 401.1 error each time a user tries to log on using this
"DOMAIN/Username" pattern, but gets in when using
"DOMAIN/MachineName/Username" Why does it need the machine name? On our test
server (using the same, *well, we think* configuration) it works, but on the
production server it doesn't. What could be the cause and/or solution?

Thanks

ThunderMusic