From your previous post:
I would set them as follows:
keyfile=/etc/letsencrypt/live/server.eyethrees.net/privkey.pem
extracas=/etc/letsencrypt/live/server.eyethrees.net/chain.pem
certfile=/etc/letsencrypt/live/server.eyethrees.net/cert.pem
or
keyfile=/etc/letsencrypt/live/server.eyethrees.net/privkey.pem
extracas=
certfile=/etc/letsencrypt/live/server.eyethrees.net/fullchain.pem
Note: This change may require the "webmin process user" access rights to those files.
If that causes access rights problems that can't be overcome:
You can copy the required files to a folder that Webmin does have access to.
[then modify the cert update script to copy those files whenever the cert is renewed]