It’s missing the intermediate certificate. But you already specify fullchain.pem, so that should be OK… Somehow it isn’t, but I don’t know anything about Webmin, so I don’t know why either.
Apparently, Webmin sucks and needs weird methods to work with an intermediate certificate.
You could try to concatenate privkey.pem and cert.pem to a single file and use that file as the “Private key file” in Webmin and add chain.pem to that other option? I dunno, but it’s worth a try