I resolved my issue. It looks like when you are part of three different local networks (in my case 192.168.100.x, 192.168.1x and 192.168.8.x) Multiplicity has problem with scanning and finding PCs.
I have been using version 3 multiplicity for years. No problems. So I recently bought an upgrade to version 4 PRO. I installed it on the same computers that are running version 3. Zero changes in the configuration. Private network, firewall open and... nothing. I can't for the hell of it see the secondary computer and connect to the primary. Has anyone had a similar problem?