Payment not going through

We’re having an issue where the payment doesn’t seem to be going through to Stripe. We have an error in the JS console:

Uncaught SyntaxError: Unexpected end of JSON input
    at JSON.parse (<anonymous>)
    at Object.success (wpep_free_payment_form.js?ver=3:174)
    at c (jquery.min.js?ver=3.5.1:2)
    at Object.fireWith [as resolveWith] (jquery.min.js?ver=3.5.1:2)
    at l (jquery.min.js?ver=3.5.1:2)
    at XMLHttpRequest.<anonymous> (jquery.min.js?ver=3.5.1:2)

When we console.log what the script is trying to parse on line 174, it comes back as an empty string.

So it seems like the admin ajax call is not coming back successfully or something similar.

Happy to deploy custom code to the site to debug/fix, just let me know.