Try reinstall Webmin, original path to miniserv.pem is (in CentOS) /usr/libexec/webmin/miniserv.pem. Or simple copy file miniserv.pem from /usr/libexec/webmin/miniserv.pem to /etc/webmin/, user root, group bin, permission 600. If You have Your own SSL certificate for Your server, You can use this certificate for Webmin too. A new miniserv.pem is merged from no-crypted yourdomain.key and yourcertificate.crt. And don't forget copy Intermediate_CA_chain.cer to /etc/webmin root,bin,400.
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I have delete miniserv.pem now how to resolv problem with this error :(
Failed to save SSL options : The SSL private key file /etc/webmin/miniserv.pem does not exist or does not contain a PEM format key
Try reinstall Webmin, original path to miniserv.pem is (in CentOS) /usr/libexec/webmin/miniserv.pem. Or simple copy file miniserv.pem from /usr/libexec/webmin/miniserv.pem to /etc/webmin/, user root, group bin, permission 600. If You have Your own SSL certificate for Your server, You can use this certificate for Webmin too. A new miniserv.pem is merged from no-crypted yourdomain.key and yourcertificate.crt. And don't forget copy Intermediate_CA_chain.cer to /etc/webmin root,bin,400.