Let's Encrypt is a certificate authority that provides free certificates to enable websites to be reached via HTTPS (rather than the not-so-secure HTTP). They are fairly easy to install through the Plesk hosting control panel.

One thing about Let's Encrypt certs is that they expire after only 90 days which means they must be renewed every 3 months.

We were having lots of trouble getting them to renew on all of our Umbraco-powered websites until we found the trick: Add the folder that Let's Encrypt looks for to the list of reserved folders in Web.config.

In the section of Web.config, find the setting for "umbracoReservedPaths" and add "~/.well-known" to the list. Like this:

<appSettings>
  <add key="umbracoReservedPaths" value="~/umbraco,~/install/,~/.well-known"/>

This resolved our issue and we have found joy once again.