Backup won't connect to a NetCenter NAS

When setting up backup under Backup Status and Configuration->Set up
automatic file backup, I select On a network. Next I enter the UNC path I
want to save the backup to. When I press Next, I get an authentication
dialog. When I enter the username and password, I get an error dialog with
the following message, "The network share could not be accessed for the
following reason:

No mapping between account names and security ID's was done.
(0x80070534).

Please insure that the network location is valid."

The NetCenter can't be joined to the domain, but it does allow specifying a
workgroup. That is set to match the domain. The same logon credentials work
when connecting through Explorer.

Anyone know why authentication fails?

Bruce