If you're getting blank window, you probably have some 3rd party extension or custom code which is not performing well. You have few options:
1. Find an exception in error logs, try in var/log/exception.log, and paste it here in order to debug.
2. Turn on developer's mode by opening index.php and un-commenting:
ini_set('display_errors', 1);and by adding following line near the line from above:
Mage::setIsDeveloperMode(true);
That should allow you to see error directly on screen.
3. Turn off 3rd party extensions, one by one. After each one extension that you've turned off, clear the cache and check if Transactional Email are working. That's how you can identify bad extension / code.
I hope you're having local/staging environment, so you can test it without headaches.
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