If you are running Build 1353 or higher, make sure you have Roon Server running before you start Roon. Roon separated the Server (RoonServer.exe) portion from the user interface (Roon.exe) in the past month (November 8 for Build 1353):
Roon also released an update, Build 1357, today:
Do you have Windows firewall exceptions set to allow roon.exe, raatserver.exe, and roonappliance.exe on the PC you are using for Roon Server?