To help us distinguish between a known cloud-side display glitch and a hard configuration failure, could you please clarify a few details?
Is ARC actually failing to play music?
Please try turning off Wi-Fi on your phone and opening ARC. Does it fail to connect entirely, or does it work despite the error message in Roon Settings?
Troubleshooting steps:
If it is failing to connect, have you attempted any manual configuration yet, such as enabling UPnP or setting up a manual Port Forwarding rule?
VPNs:
Are you running any VPN software or specialized firewall apps on your Core machine?
There was a brief upstream cloud outage with Roon services around the time you posted, which interrupted port forwarding for many users. This issue has since been resolved.
Please share a screenshot of your Settings → ARC page within Roon if you’re still experiencing issues with UPnP.