Hint:
Enable error messages so that they are displayed!

See also: Why aren't error message displayed in PHP?

To fix this please proceed as follows:

Look for the variable DOCUMENT_ROOT in the xtCommerce scripts (in many cases labeled as $_Server['DOCUMENT_ROOT']) and change it as follows:

After installation look for the abovementioned variables and adjust them (when existing)

This is just necessary once.

According to customer feedback this mostly affects: 
application.php and install_step5.php.

Make absolutely sure to adjust all of the abovementioned variables. Otherwise there usually will occur an error during step 5 / 6 because a corrupt configuration file is being written.

Furthermore there should be made modifications in the files "includes/configure.php" and "admin/includes/configure.php":
define('DIR_WS_CATALOG', '<Directory path (starting from www/data/)>');
define('DIR_FS_DOCUMENT_ROOT', '/home/<ftpusername>/www/data/');
define('DIR_FS_CATALOG', '/home/<ftpusername>/www/data/');

Moreover it can be necessary to make the following tiny modification in the file 
xtc_installer/includes/application.php durchzuführen, da das Script das DocumentRoot nicht erkennt.

Changes are highlighted RED.

old: 

if (!defined('DIR_FS_DOCUMENT_ROOT')) {
      define('DIR_FS_DOCUMENT_ROOT', $_Server['VirtualDocumentRoot']);

new:

  if (!defined('DIR_FS_DOCUMENT_ROOT')) {
//      define('DIR_FS_DOCUMENT_ROOT', $_Server['VirtualDocumentRoot']);
      define('DIR_FS_DOCUMENT_ROOT', '/home/<ftpusername>/www/data/');

For maximum performance we recommend using xtCommerce with mySQL 5.x