When trying to add a server which uses a Letsencrypt cert in Cluster Webmin Servers, I'm getting the following error:
Failed to connect to xxxxxxx.com: Invalid SSL certificate : Certificate is signed by an unknown CA : /C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3 (code 21)
Everything works fine when "Check remote SSL certificate?" is unticked for the server in Webmin Servers Index, so it seems to be an issue with validating the SSL cert. I'm using this workaround for now, but it would be good from a security point of view if the validation issue can be fixed.
The same issue has been reported by another user here: https://virtualmin.com/node/55182