This is a fresh server, with webmin freshly installed-- in fact, installing usermin is the first action I'm (trying to) take with it. I'm logged in with a user that has sudo privileges, however I've also tried it as root.
I select Usermin from the list of unused modules, then "Install Usermin tar.gz package". This brings me to an error screen that says "Expecting form-data encoding, but got normal encoding".
The lack of help I've been able to find for this is disheartening. Either no one uses Usermin, or no one has ever had this error. In fact, "usermin" isn't even an existing tag here.
Trying to install virtualmin gives a different error: Failed to install module from http://download.webmin.com/download/virtualmin/webmin-virtual-server_5.04.gpl_all.deb : Not a valid module file : tar: This does not look like a tar archive tar: Skipping to next header tar: Exiting with failure status due to previous errors.
To arrive at that I followed the install instructions: Go to Webmin Configuration-> Webmin Modules-> Install from ftp or http URL.
Neither usermin or virtualmin are critical for me. I've never used them before, but I'm setting up a new server today, and thought a graphical UI might make my life a little easier. Webmin itself installed just fine.