I too am now having the same "user authentication" message on the mac side.
I just upgraded from 5.5 to 5.6. It was working before, now not.
I recall doing something before to make it work. I can't remember what it was, but I think it was a module that had to be enabled. Something to do with the password file.
I am trying to remember now, but it was a couple years now.
Sorry to post a non fix.