[Satellite6] Capsule synchronization fails with "Host did not respond within 20 seconds" error, despite katello-agent is running fine

Issue

• an attempt to synchronize external capsule fails with error "Host did not respond within 20 seconds. Is katello-agent installed and goferd running on the Host?"
• katello-agent / goferd service is running on the capsule and has several established connections to Satellite's port 5647
• goferd is even configured with heartbeats