Yeah, 3rd person indeed. There must be an issue with webmin.
I wouldnt recommend using certbot however, only because, in my experiance, as soon as you manage certs with certbot, outside of virtualmin its very difficult to get virtualmin to manage them for you again. I found the easiest way was to reinstall.
If you can take a look at my thread, and also my post i made in the 2nd person who has had this problem and try those steps to see if the problem is resovled for you that would be helpful. Might be we need to build a picture on this problem and raise a bugfix if it is indeed a webmin specific issue.