Oh, for heaven’s sake! I found the problem as I was in the process of building a reply to your last question.
I’d included both the non-SSL and SSL versions of the VirtualHost block in the new .conf file, but the SSL version was malformed (which makes me wonder why Apache tolerated it, but that’s neither here nor there): I left out the
Include /etc/letsencrypt/options-ssl-apache.conf
SSLCertificateFile /<path>.pem
SSLCertificateKeyFile /<path>.pem
and the </IfModule>
at the end, after the </VirtualHost>
!
I fixed that, and tried the command again, and this time it worked fine.
THANKS!