Let's Encrypt have identified a security issue with their TLS SNI verification method and have disabled it while they investigate. Our Load Balancers use the TLS SNI verification method so currently cannot create or renew Let's Encrypt certificates.
Let's Encrypt expect that have this solved by Friday 12th January. Currently no overdue renewals are pending, so no existing Load Balanacers should be affected. Creating new load balancers with Let's Encrypt certificates will currently not complete verification.
If you need to build a Load Balancer with SSL/TLS support, you'll need to purchase a certificate and provide it manually.
More information is available on the Let's Encrypt status page
Update, 11th January 17:40: We've been whitelisted by Let's Encrypt and verifications are now re-enabled. Renewals and new certificate creation is now working fully.