I was able to verify that this happens only with OS X 10.5.x
Virtualmin can not create new servers, and it shows the following error:
Creating administration group domainnamehere …
… failed to create administration group!
Failed to create virtual server : Critial feature Administration user was not properly created - Virtual server creation halted.
This happens because the Users and Groups Webmin module is broken on OS X 10.5.x:
No users were found on this system!.
No groups were found on this system!.