I'm using Cloudflare and getting a 503 error

A 503 means the service is unavailable.

If you are using Cloudflare and have DDoS protection enabled, Cloudflare will attempt to use JavaScript to validate the browser is legitimate. The HTTP client used by the server is to send API requests, and therefore does not have JavaScript. You will see a 503 error logged in your console, and API authentication will fail.

To solve this, you will need to whitelist your server's IP address in Cloudflare's IP whitelisting feature to stop the check affecting the MineSync API connection.

For more information, please read Cloudflare's article on whitelisting IPs in the firewall.