I have an SSL certificate from LetsEncrypt. My website successfully uses HTTPS with it.
However, at random intervals this suddenly stops working. I get a Common Name mismatch error. It then just as suddenly starts working again a minute later.
A "Common Name mismatch error" means that the URL shown in the address bar does not match the URL that the certificate is registered for. The most common reason for this error is creating a certificate only for domain.com and then accessing your site with www.site.com. With LetsEncrypt you can add additional alternate names to your certificate so that any URL that will be used to reach your server can be included and prevent this error.