PROBLEM:"Failed to upgrade from www.webmin.com : You are already running the latest version of Webmin"

Even if you actually have the latest version installed.

Run this command on SSH:

miniservconffile=$(find / -wholename "*web*/miniserv.conf") ; rootdir=$(grep "^root=" "$miniservconffile" | sed -e 's/root=//g') ; echo "2" > "$rootdir/release"

It basically creates the file /usr/share/webmin/release and writes 2