WiFiSSLClient will not connect to servers with LetsEncrypt certificates
The solution is to update the certificates the WIFI 101 Firmware Updater tool (in the Arduino IDE).


Install the FirmwareUpdater sketch, as described on the Arduino site.
Select Tools -> WiFi 101 Firmware Updater. Click on the Add domain button at the bottom, enter your domain and click Upload Certificates to WiFi module.

Upload the sketch to your Arduino. It should now connect to the server.