I disable IPv6 everywhere to simplify things, certainly hasn't made things worse. I also added host entries to each workstation binding to the correct IP. The issue now shows itself when, from time to time, some workstations cannot ping the unRAID hostname, cannot access shares or GUI, etc, at the exact same time other workstations can ping it just fine, all hitting the same IP, eventually this works again.
This box and IP and MAC worked perfectly with Windows for years.
I'm utterly baffled.