Multiplicity has been working fine on my Win 10 PC (with 1903 installed a couple of weeks ago) until I rebooted today.
The icon in the system tray is grayed out, and indicates 'Disconnected'.
I've uninstalled and reinstalled Multiplicity on both primary and secondary PC's and both have most recent updates.
It shows as active in the system tray of the secondary PC.
Windows Defender firewall settings are allowing Stardock on both PC's (private only).