Let's Encrypt won't follow redirect to URL with port 10000

[calport]

So you need to solve this problem: do something to get http://admin.example.com redirected to httpS://example.com:10000 instead of applying for a SSL cert for httpS://admin.example.com:10000

Addendum: See this

How to setup new subdomain for email management?

This operation can be carried out via the Virtualmin GUI. To redirect webmail.domain.tld to /roundcube folder: Virtualmin --> Services --> Configure Website. Click Edit Directives, find the lines which say RewriteCond %{HTTP_HOST} =webmail.domain.tld RewriteRule ^(.*) https://domain.tld/20000/ [R] Change domain.tld/20000/ to anything you want, e.g. domain.tld/roundcube/. You must restart the web server after this. You may also wish to do the same for Virtualmin --> Services --> Configure SSL…