One of our servers (Ubuntu 18.04 with Nginx) is using Let's Encrypt's certbot to renew its SSL certificate regularly via script. Recently, it reported in web browsers as having an expired certificate. When I ran
it showed as having the certificate set to expire months from now.
Just on a lark, I rebooted the server, and then it was fine, and the web browsers showed the new certificate. Usually, a reboot isn't necessary. I'm not sure why it was all of a sudden this time. But just FYI: if you're using Let's Encrypt to renew your site's certificate, and it's definitely renewed but randomly not showing that way to client machines, try a reboot.