So the secondary validation server can find your ip address, there is no timeout (blocking firewall) reported - but there is a http status 503 - Internal ServerError.
May be your proxy has sometimes a problem. Or you don't have a proxy and it's only a temporary 503 of your main server.
That's a very good observation, @JuergenAuer. Any idea why the primary would succeed and the secondary would fail? The OP actually posted the same issue with another domain name, but the topic got hidden due to strange editing. As a leader you may still be able to view its history: