Plugin not adding emails to subscription list

Hello,
I set up a separate page with the shortcode in and linked this page in Settings – Appearance.
I have also linked the widget to the same page I set up.
When I enter an email address I get the error message ‘Sorry there seems to be an error on the server. Please try later’.
I have taken over the management of this plugin and notice that it must have been working at some point as there are subscribers showing in the plugin settings.
Any ideas what I am doing wrong?
Thanks

https://www.ads-software.com/extend/plugins/subscribe2/