The instructions how to install Webmin on Debian (and thereby also Ubuntu) seem a bit outdated because edits to the file /etc/apt/sources.list should be replaced by adding a file to the directory /etc/apt/sources.list.d/. So, instead of adding
http://www.webmin.com/deb.html to the file /etc/apt/sources.list create a new file /etc/apt/sources.list.d/webmin.list with that content and possibly a comment why