check /etc/apache2/conf-available/z-ocsinventory-server.confPerlSetEnv OCS_DB_USER ocswebPerlSetVar OCS_DB_PWD “Password”and verfy that this was installedphp-xml
also check this site
https://kifarunix.com/install-ocs-ng-inventory-on-centos-8/
quote "In order to avoid issues as we have experienced with other versions of PHP like OCS-NG blank screen, OCS-NG stuck on Processing… with PHP 7.3, this guide will stick to using PHP 7.2 which is the default available version on CentOS 8"
Kind regards