Hello!

To allow Sidecar through macOS's "Block all incoming connections" firewall, ensure you've correctly added com.apple.sidecar-relay and com.apple.sidecar-display-agent to the allow list in Firewall Options. If it stopped working, verify these bundle IDs are still active in Activity Monitor and consider related Apple processes. Temporarily disable the firewall to test, check firewall logs in Console, and ensure both devices are on the same network. You might also try resetting firewall settings and re-adding the rules, as the initial success suggests the bundle IDs are likely part of the solution, but something else might be interfering or has changed.