Ghostdare is correct in that the simplest course of action is to delete index.html.
He's correct also in that Apache looks for index files in a particular order (index.html is the first, by default).
directive controls the order. You can either change this value server-wide (e.g., insert the new value into /etc/apache2/httpd.conf
) or for individual sites via the ISPConfig interface (Sites -> example.com -> Options, Apache Directives box).
An example would be to paste-in the following:
DirectoryIndex index.php index.shtml index.html index.htm
Apache looks for the files in order from left to right.