Can anyone shed any light on why, after cumulative update version 21H1 for x64-based Systems (KB5003637) installed 2021-06-08, I lost all connections on my LAN?

I have five discoverable devices on the LAN, four PCs, two laptops (all running Win10 pro) and two Synology NAS's. All are connected through ethernet cables using fixed IP addresses with Netbios disabled on all of the network adapters. With the exception of the two NAS's, all of the PCs and laptops were discoverable by each other (WSD discovery) and could connect to shared content on each other. I have always had issues with the two NAS's and so connect to these using their IP addresses.

The two laptops and three of the PCs were updated more-or-less at the same time. After their update, all connections were lost except to the PC for which the update was set to be installed overnight. The next day, its visibility to the other devices had disappeared and it showed no connections.

Any help gratefully appreciated

Windows version: 21H1 (OS Build 19043, 1052)