webmin gets downgraded by apt
Using the domain controller vmdk image in virtualbox. I logged on and first thing I did was hit the webmin interface which required an update to 1.500. I hit the Upgrade Webmin Now button and it seemed to upgrade fine. I was able to continue using webmin 1.5. ok. However later on I used shellinabox to get a console prompt and use standard apt-get update/upgrade to upgrade the whole machine. webmin got downgraded back to 1.490.
Perhaps you need to disable the web upgrade button in webmin?