On April 6th we made a fix on how we generate Let's Encrypt certificates due to an API change LE made on April 24th (today). As it turns out our changes where broken and resulted in certs not getting generated over the last 2 weeks.
The system is fixed and will be pushed to all servers in the next 24 hours, we have made the changes live on N9 and N12 to do testing.
Apologies for the error.