2
Search the files for "xoopstree.php".
That file should be loaded using
include_once XOOPS_ROOT_PATH."/class/xoopstree.php";
or
require_once XOOPS_ROOT_PATH."/class/xoopstree.php";
But it might be getting loaded using
include XOOPS_ROOT_PATH."/class/xoopstree.php";
or
require XOOPS_ROOT_PATH."/class/xoopstree.php";
The "_once" ensures that the file will only be loaded once, which avoids that "Cannot redeclare ..." error.