Discount code link not working

Hi,

I am using Paid Memberships Pro to sell training courses (with WP CourseWare) and in the Checkout Page (before to pay), where user starts subscribing, the link to add a discount code does not open at all. It seems to be an AJAX call and some errors appear in the page where the ajax seems to be returned in HTML (unexpected token <).

Do you have an idea what could be the issue?

Thank you.

https://www.ads-software.com/plugins/paid-memberships-pro/