I've been using Multiplicity successfully for several weeks, with a very simple configuration. The primary is a Windows 10 desktop PC and the secondary is a Windows 10 laptop.  I don't believe anything has changed, but this evening the secondary is invisible to the primary.  On the primary configuration grid the secondary is greyed-out.  Clicking the gear wheel on the 'Configure' pop-up delivers me the message "Please check the firewall settings on the secondary (should not be set to Public /Guest), the computer name and the IP address"

Before now I haven't touched the firewall settings on either machine for as long as I can remember. To try to fix this problem I've turned off the firewalls on both machines and tried various combinations of restarts.  Nothing works.

I'd be grateful for any sensible suggestions as to what I could try next.