I described this as a reply in another thread, but I'll detail it as it's own thing here.
I have a setup with a Windows domain running, so I need to use the DOMAIN\username style authentication. I was not having any trouble, so I thought I would check my server security logs and see what it was showing. I noticed several entries that appeared to be saying that my server was receiving the domain and username in the wrong fields. i.e. the username was being submitted as the domain, and the domain as the username. Sure enough, when I flipped them around in the dialog, I was able to auth successfully. i.e MYDOMAIN\username didn't work, but username\MYDOMAIN does. That seems like a bug to me.
I haven't tested this with just a plain WIndows PC sharing a folder to see if it affects that as well, but I wouldn't be surprised if it happened there as well. Testing this is a big difficult because of the lack of caching the credential.
Thank you!