Please see these recent discussions which are related to the issue you mention:
I always create a virtual server with the same domain name I use as hostname - (e.g. vps01.indiax.com), apply SSL to it (mostly via Let’s Encrypt) and then use the “Copy to Webmin” etc. group of buttons to apply that SSL to ports 10000, 20000, Postfix, Dovecot at al.
I would be happy to help you in any way I can, bro! Your community is doing a great service to the nation.