I’m having the same problem. iPhone 6s and iPad 5th Gen (2017) running iOS 14.4. Updated iOS apps first then Core to 1.8. Same issue, same error message.
Core is Roon Server running on a headless MacMini connected by Ethernet so WiFi is off, but WiFi connection from MacBook Air and wired connection from desktop Mac are both working. Network settings and firewall check out okay.
This Knowledge Base article says to confirm Core is set to accept remote connections, but that is not an option for me in >Settings>Setup.