- Verify that my SSL cert was properly configured.
- Verify that FreshRSS API access and API password were set up.
- Verify that all details were entered accurately in the mobile apps.
- Turn off Wi-Fi and try refreshing over LTE. This worked, indicating that the issue is not the client/server app configuration, but the network configuration.
- Reset iPhone network settings and re-join Wi-Fi. Errored again.
- Change DNS settings on my Wi-Fi connection to use Cloudflare resolvers (188.8.131.52, 184.108.40.206) instead of Spectrum. This worked (update: temporarily. It also works over VPN.)
Based on my troubleshooting, I suspect Spectrum is doing something funky with DNS or SSL. (Possibly related: The Spectrum TV app is in Day 2 of an extended outage that started around 06:00 UTC -4, 8 June 2022).