I have one domain for which I'm trying to enable SSL. When I Edit Virtual Server and click the box for "Apache SSL website enabled?" and hit save, I get:
Creating SSL certificate and private key .. .. SSL website failed! : Failed to open /home/thedomain/ssl_certificates/cert.pem.webmintmp.6408 : No such file or directory at /usr/share/webmin/web-lib-funcs.pl line 1433, line 1.
I'm running Virtualmin 6.00, Webmin 1.851. I have several other domains running SSL fine (though none of them have a /home/domain/ssl_certificates directory).
I also note that the domain.conf.lock file remains in /etc/apache2/sites-available dir after trying to enable SSL.
FWIW, I'm using Let'sEncrypt certs for the other domains, and I don't recall having anything special to do other than enabling SSL and going through the Let'sEncrypt menus for getting a cert.
(I posted this in the forums too, but I'm under pressure to get this working for this domain, so copied here; apologies...)