Is it possible that a firewall is blocking the connection between `cloudflared` and Nextcloud?

Yes, a local firewall on the server running `cloudflared` could be blocking its access to the Nextcloud instance, especially if Nextcloud is running on a non-standard port or using a specific IP binding. Check your server's firewall rules (e.g., `ufw`, `firewalld`, `iptables`) to ensure the port Nextcloud is listening on is accessible by `cloudflared`.