WPMU Subscribe by Email Plugin Hangs

Hi there,
I'm having trouble with the WPMU Subscribe by Email plugin. It seems to hang with the spinner when I try to submit an email address. I have tried disabling all plugins and clearing cache.
You can see the email widget on this page:
https://mumsbusiness.club/freebies/ (under SUPER SUBSCRIBERS)
Interestingly, the follow tab to subscribe at the bottom works

Can you please take a look for me? - I have granted support access