After allowing a Windows 10 Pro system to use Windows Update to apply the 1511 version upgrade (build 10586.3), my Hyper-V Virtual Switch failed with an error reporting "Static MAC address 'B8-CA-3A-7F-97-F8' for 'vEthernet (New Virtual Switch)' (017CBB32-2B9B-49F7-B257-3614649BAF21) conflicts with another adapter." although the error I saw was less specific. (The message here is from the Microsoft\Windows\Hyper-V-VMMS\Networking event log in Event Viewer > Applications and Services Logs.)
I dropped the existing switch and created a new one.
That gave me Internet (and local TCP) access, but local UNC paths and related infrastructure fails. (UNC paths and mapped drives for a VPN now worked.)
What should I try next?
There are related questions, but most are referring to total networking loss.