In the configuration modal for the button (in the formbuilder) there are three actions the user can choose:
 * Redirect to a Content item
 * Redirect to an URL
 * Show a message

The "submit" button for this modal won't work unless if user chooses Redirect to a Content item or Redirect to an URL. Instead, the modal will just load again

In order to work around the problem, the user must first select Show a message, then provide a dummy message. Then he can select Redirect to a Content item or Redirect to an URL and the submit button will work just fine