This looks like you didn't had libxml2-dev installed when you installed ISPConfig.
what is the output of
Code:
sudo dpkg-query -W -f='${Status} ${Version}\n' libxml2-dev
If it's not installed you will need to install it
Code:
sudo apt-get install libxml2-dev
and then either recompile ISPConfigs PHP with XML support, or, the simply method, reinstall ISPConfig. Since 2.2.24 ISPConfig detects if libxml2-dev is installed and if yes it enables XML in the bundled PHP.