Does this same behavior occur for all your audio endpoints? If you try starting playback on “System Output”, does that work as expected?
Can you share more information regarding your network setup? What is the model/manufacturer of your router? How is your Core connected, is it connected via Ethernet to your router?
Just to confirm, the only media location you have is the QNAP, you don’t use any streaming services in this setup? Is the behavior the same if you reboot your Core, Networking gear and QNAP? If you copy a few files off the QNAP and save them on the iMac’s HDD, do those play as expected?
Rebooting everything worked. I double checked my network config and realized that the iMac (Roon Core) is connected to the QNAP NAS through a DLINK Ethernet switch (DGS-108) and does not go through the Asus router.
I suspect rebooting the unmanaged DLINK switch was likely the key to getting everything working again as it is usually the last thing I think about rebooting. But who knows, if technology worked as advertised many of us in IT would be out of work.
You may not always be appreciated but you do good work and your support has always been timely and helpful.