I found an error in the subscription form's HTML.
The label's "for" attribute is empty. Also the input field has no "id", which is need to link a label to a specific input field.
I am running WP 3.5.1, Wysija 2.4.4 and use the PHP method to integrate the form into my website.
A fix would be nice, as this is important for usability.
(I also wrote you an email about this. Sorry for the trouble.)