If you didn’t install using the automated install script, but did use the RPM packages of Virtualmin and Webmin, you’ve got a system state that the upgrader doesn’t know how to handle (the upgraded best deals with systems installed with install.sh, but it can be fooled into working pretty well with just about any other Virtualmin GPL system).
Here’s what you do:
Remove /etc/virtualmin-license to undo the bit of the upgrade that did occur
I, of course, assume you have good backups. Upgrades are not particularly dramatic or dangerous. But, I want to remind you to keep good backups, because something is bound to go wrong someday.