I have the same Problem.
Any ideas so far?
Does it work if you put the anonymous access back? I didn't think this was designed to handle authentication within IIS since the files within there will be accessed by devices that have no capability to authenticate to a domain.
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If anonymous access is turned on, I get a message "UNC authentication was successful. Unable to validate http authentication to http://servername/landesk/files/drivers, the server allows anonymous access."
So I have both anonymous and windows integrated authentication turned on and I get the same message. It's as if you cannot have the folder config'd with anonymous access.
[UPDATE: with anonymous on in IIS for the drivers folder, I do still get the error message under the Preferred server configuration when I try to test the credentials. But if I click Ok and save the configuration anyways, then go back to the OS deployment, it works. So I suspect that this error message about anonymous access really should be a warning instead.]