Hi Graham,
Welcome to the community. I’ve moved your topic to the support section of the forum.
Support will be along to help, but can you try a couple of things in the meantime?
Can you reboot your iMac core and see if that helps?
Does Roon work properly running directly from the iMac?
Are you running the Firewall on the iMac?
If this doesn’t help, please follow the instructions.
In order for Roon’s @support team to better assist you, please provide a brief description of your current setup using this link as a guide.
Make sure to describe your network configuration/topology, including any networking hardware currently in use, so they can have a clear understanding of how your devices are communicating.
Cheers, Greg