Within the inc.main.php file which can be found in the /home/admispconfig/ispconfig/web/roundcubemail/config folder, you can enable the RoundCube installer.
If you did that, you can enter the RoundCube installer within your browser. The Installer will tell you exactly which requirements are necessary to run RoundCube successfully.
The current ISPConfig version (since version 2.2.24) compiles PHP automatically with libxml2 support if you have the libxml devel package installed on your server.
Probably you need to install the libxml devel package form your Linux distribution and upgrade ISPConfig to the latest version.
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