Windows machines lost access to each other

I upgraded Windows 10 pro server, rebooted and, while server is working fine, it completely lost connectivity to any other ZeroTier-connected machines on the same network.

I reinstalled ZeroTier on 2 Windows boxes, downgraded, upgraded - to no avail.
Another machine also lost connectivity to these two.

3 Windows pro machines that used to connect to each other fine - completely lost connectivity between each other.

What could I do to remedy this?

Your post lacks the details required to help debug your problem. Are all nodes showing up in the controller as online on the expected network? Can they discover eachother e.g. ping the broadcast address, wait 5 seconds, run zerotier-cli peers? What’s the state of the ARP and NDP cache on the nodes? Can you capture packets on the ZeroTier interface(s) with tcpdump?