Sign ups are on a daily basis. When sign ups are full, they reset on midnight UTC. Then you can sign up. After Johnny is more stable, he will have unlimited sign ups per day.
I'd bet it has to do with the cert nearing expiration, especially if it just recently started. I know AutoSSL certs are usually renewed/replaced once they're <14 days from expiration... Try renewing your certificate and see if a fresh one still does it.