Contrary to the manual, PHP’s
get_included_files() does not ignore includes from the
auto_prepend_file directive. This is true on my version, 5.1.1 and on 5.2 according to this comment:
http://php.net/manual/en/function.get-included-files.php#73541
This means that if you have auto_prepend_file set, when MODx’s index.php checks if it was included itself via count(get_included_files()), the script exits showing a blank page.
Until 0.9.6.1 is released, I’ve put up a rough workaround here:
http://modxcms.com/bugs/task/877