The error "An interplugin communication error occurred" is displayed when instances of smart:EQ3 cannot communicate with each other. This is usually caused by firewall settings or a VPN.
If you are running Windows on your machine make sure the port UDP 5670 is not blocked. Therefore open the Windows Defender Firewall with Advanced Security and go to Outbound Rules. If the port is blocked create a new rule for UDP port 5670 which allows connections.