Sounds exactly like you’ve got Webmin installed from somewhere other than Webmin.com or Virtualmin.com (maybe an old Debian package). The Debian official Webmin deb was simply horrible…it never worked right. The install.sh tries to remove old versions, but maybe something went wrong, or you have some apt sources configured that claim a newer version than ours. Not sure. Or maybe a tarball install?
If it’s not a production system, we could just drop back to zero and try the install again–making sure that Webmin is uninstalled and won’t be installed from some untrustworthy source.
Actually, though, it wouldn’t be just trying to use Apache 1.3 syntax in that circumstance…it would be trying to use Apache 1.3 everything. Anyway, something is broken in the /etc/webmin/apache/config, and the simple way to fix just that would be to copy /usr/share/webmin/apache/config-debian-linux-3.1-* to /etc/webmin/apache/config and restart Webmin. That’ll probably straighten this particular issue out…but if we started with an odd Webmin installed, I wouldn’t trust it.