I have just upgraded to Fedora 18 which included an upgrade to Apache 2.4.4 web server. In the httpd.conf there is now a directive to load files from a directory but to keep on going if there are no files there. It works the same as Include, but does not fail.
The current version of the apache module in WebMin does not handle this directive at all. It knows it is there but does not follow it. As my VirtualHosts are in this directory and loaded that way they were not showing up. Changing the directive to Include makes it work.
The IncludeOptional directive needs to be included in the valid commands in the httpd.conf file. Here is the section from the file:-