[jillelaine]

Thanks to the above suggestion, I discovered that my $_SERVER["DOCUMENT_ROOT"] was set to /usr/local/apache/htdocs and not /home/user/public_html, where the website is located.

So the original configuration lines in udm-style.php and udm-dom.php, $config = $_SERVER['DOCUMENT_ROOT'] . '/udm-resources/udm-custom.ini'; meant that the server was looking for the udm-custom.ini file in /usr/local/apache/htdocs/udm-resources/, which was incorrect.

I commented out the original configuration line and wrote:
$config = '/home/user/public_html/udm-resources/udm-custom.ini'; in both the udm-style.php and udm-dom.php, saved and uploaded to the server, and, pow, that worked!

THANK YOU! Hope this information helps someone else.