Automatic index.html doesn't work but index.php does

If I make a directory and slap a index.html file in it, I'll just get a page not found when going to domain.com/directory/. The same goes for index.htm.

If I try the same thing with a index.php file (domain.com/directory/index.php) it works great.

It also works if I specify the full name, domain.com/directory/index.html.

I have checked and this is in the Virtualmin config: DirectoryIndex index.html index.htm index.php index.php4 index.php5

I also see the DirectoryIndex line in /etc/apache2/sites-enabled/...

What else should I check to see why index.html and index.htm doesn't work?