A letsencrypt certificate by itself isn’t always accepted by a lot of browsers.
Your .crt file needs to contain the full chain of authority for your certificate.
Did you create the .crt file with your certificate + private key or did you include the chain.pem file, too?
Your letsencrypt directory should have a fullchain.pem file.
Start with fullchain.pem and append the privkey.pem to create your .crt file for Xojo.