Since October 1st 2021, a root certificate of Let's Encrypt expired and is causing trouble for those who have not updated their systems in time. That the certificate was going to expire was known long before and Scott Helme dedicated a blog to the issue called Let's Encrypt's Root Certificate is expiring!. Now that October 1st has passed we are finding out that a lot of people are not keeping their systems up-to-date, as we see an increase in the support requests regarding this issue. The error message is the same for everybody The certificate issuer's certificate has expired. Check your system date and time. In the Joomla forum there are also users experiencing the same issue and a solution is provided in this forum thread. Applying that workaround should make your site run again.