Subscribe by Email - not showing subscribers or allowing subscriptions

We're having a few issues on all our sites and the Subscribe by Email plugin.
1) If you're using the plugin widget and you attempt to subscribe while not logged in the icon spins indefinitely and no subscription ever takes place
2) When we look at the list of subscribers in a particular site we see none listed, but if we go and look at the wp_xx_subscription tables in the db they have many subscribers listed.

We have tried rolling back the plugin to version 2.75 on our test system in the hope that would change things following other forums found online as well as removing the plugin and reinstating it also with no change in behaviour.
I have also tried testing this plugin with a variety of themes.

I have noticed the following error message when trying to subscribe an email address via the widget:
Timestamp: 4/02/15 3:13:35 pm
Error: TypeError: return_data.data is undefined
Source File: ..../wp-content/plugins/subscribe-by-email/assets/js/widget.js?ver=4.0.1
Line: 47

Many thanks for any help you can provide,

Kimberley