I just tried to install Virtualmin GPL on my VPS (1.6.32-5-openvz-amd64), where is running Debian Squeeze and I’ve got this error: Failed to detect operating system.
In the /etc/issue is: Debian GNU/Linux squeeze/sid \n \l
It’s possible that your provider made some changes to their Debian image… that’s not what you’d typically see in the /etc/issue for a Debian 6 installation.
However, one thing you could do is to edit your /etc/issue file, and change it’s contents to read:
Debian GNU/Linux 6.0 \n \l
That should fool the installer into thinking it’s a typical Debian 6 install… from there, we just have to cross our fingers and hope nothing else about the install is out of the ordinary