The "During secondary validation" means the primary Let's Encrypt servers were able to reach you but one/some of its secondary sites could not.
Does this problem repeat? Sometimes temp comms problems can cause this.
There was a recent change in that Let's Encrypt has added a 4th global validation point. But, I don't think this is causing the above problem. Just be sure that your firewall allows HTTP access from any IP / geographic region and you should be fine. I see you have recently renewed some of your other certs which is why I say I don't think it it cause of above.