Shortcode Ultimate seems to break the signup form of the latest MailPoet (2.6) version:
- Install MailPoet (http://wordpress.org/plugins/wysija-newsletters/) & Shortcode Ultimate
- Create a new signup form in Mailpoet and add it to a page (using the shortcode or iFrame)
--> An error message appears on the page if no user is signed in
--> After every entered valid email address an error message appears that email address is invalid.