Shortcode only saying "Loading…" upon subscription attempt

I’m using plugin version 3.1

I have sent an email with the same problem and url via your website earlier today.

When people press the subscribe button, the page doesn’t refresh, it’s stuck at loading…

This happens when using the Single Opt In option. When I use the Double Opt In option I get the popup with the check your email message.

In both cases the email and name are registered. It’s just that the page doesn’t refresh after clicking the Subscribe button.

In the Console I get the following errors:

Refused to set unsafe header “Content-length”es_submit_requests @ es-widget-page.js:65
es-widget-page.js:66 Refused to set unsafe header “Connection”es_submit_requests @ es-widget-page.js:66

Refused to set unsafe header “Connection”es_submit_requests @ es-widget-page.js:66
es-widget-page.js:82 Uncaught TypeError: Cannot set property ‘innerHTML’ of null

Uncaught TypeError: Cannot set property ‘innerHTML’ of null @ es-widget-page.js:82

When I comment out the first two lines the first two errors are gone.
When I change line 82 into
window.onload = function() {document.getElementById("es_msg").innerHTML = es_widget_page_notices.es_success_message;};
the last error is gone, but it’s still stuck at loading… after clicking the Subscribe button.

I would appreciate when you have a look at it.

Thanks

https://wordpress.org/plugins/email-subscribers/