When creating a domain in version 2.80 and 2.81 Apache configuration gives the above error--even though Apache is running,and Virtualmin knows it is. Stopping and starting Apache from the Virtualmin interface works fine.
You can see this problem in action on my CentOS4 test Xen system:
Using the root password we've been using for all of our test systems. This Xen instance should be up and running for the next day or two, at least.