My certificate should has been valid until October 2017. However, when I tried to re-download it using acme-client, no valid certificates for my domain were found. It's strange that there were no notifications from Let's Encrypt either. They always send emails before my SSL certificate expire. Maybe I should contact their support as well. I renewed my Let's Encrypt certificate and installed it back without any issues. That may be the cause. Could my certificate became invalid if you tried to generate another Let's Encrypt certificate for my domain? Or if you tried to update certificate for webmail.trespasser.eu.org, cpanel.trespasser.eu.org, etc, while I am using SSL for trespasser.eu.org and www.trespasser.eu.org only? In addition, there were .well-known/acme-challenge directories in my domain folder. But I always delete them after domain verification is completed.