I must say, this one has me stumped. The only remaining suggestion I've got, given that you have no debug-logs in your Zen Cart's /cache folder (since you're running v1.5.0), is to edit /includes/application_top.php and change the line that reads
Code:
define('DEBUG_AUTOLOAD', false);
to
Code:
define('DEBUG_AUTOLOAD', true);
Refresh the page that's giving you a whitescreen to see how far in the initialization process you're getting before the code crawls off into the sunset. Make sure to change the value back to false once you've gathered the information.