This is a problem about the interactions of your hosting configuration with the
acme_tiny used by Webmin. I understand the nature of the problem, although I’m not sure exactly who is most to blame here.
Your site is currently set to redirect http://videos.nightlifeporn.com/xyz to https://videos.nightlifeporn.com/xyz (for any value of
xyz). That’s normally fine and even a good practice, but right now the HTTPS site doesn’t work yet from the point of view of most software because the certificate is invalid. Therefore, when acme_tiny goes to check whether this validation file was created properly, it complains that the certificate is invalid, and believes that the validation challenge wasn’t solved correctly.
But this is a mismatch between the behavior of acme_tiny and the Let’s Encrypt validator, because the Let’s Encrypt validator, as a special exception, will ignore certificate validation errors when downloading validation challenge files after following an HTTP-to-HTTPS redirect. Therefore, acme_tiny is being too strict in its check. The challenge is valid in this environment, from Let’s Encrypt’s point of view, and should not be rejected as invalid by client software like acme_tiny trying to predict what Let’s Encrypt will think.
One workaround for you could be to temporarily disable the HTTP-to-HTTPS redirection behavior on your site; a better fix might be to get acme_tiny to be less strict in this specific check.
I think you’ll need to ask one or more of your hosting provider, the Webmin developers, or the acme_tiny developers to make these more permanent changes.