when creating a new subscriber and hitting the subscribe-button, I do get an empty “subscribe.php” page with no content. In the subscriber/Massive Changes-tab I see that subscription was successful. I also tried to link to a custom confirmation page but same result. This problem appears either in single- and double opt-in.
I saw the problem, but I cannot understand why from outside. If you can provide me an administrative access I can add some debug code.
But before, enable the log at debug level on diagnostic panel, try a couple of subscription and send me the log files you can find on wp-content/logs/newsletter.