[Let's Encrypt] Can't create SSL certificate for mail.domain.tld

Hi!

I have two domains currently configured in my Virtualmin server. The first one is working correctly including using SSL certificates by Let's Encrypt for domain.tld, www.domain.tld and mail.domain.tld (default options in Virtualmin)

The problem is with the second domain. I can't request the SSL certificate for mail.domain.tld using Let's Encrypt. An error always occur:

It's a 404 error. How is this possible? This worked like a charm without problems for my first domain virtual server.

As a test, I created a "test.txt" file on that location, and tried to access it via browser, and the server throws a "404 error not found".

Virtualmin configured incorrectly my second domain virtual server? How to resolve this?

Thank you!