Mixed Content: The page at '<URL>' was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint '<URL>'. This request has been blocked; the content must be served over HTTPS.

The error generally occurs when you have a HTTP asset being served in a website with HTTPS connection. Please note that modern browsers do not allow your webpage to make an HTTP request if the content is being served over HTTPS. I would analyse the resource located at the <URL> and try to serve it over a HTTPS endpoint. You can also front your website by CloudFront which will allow the content to be served via HTTPS to the end-user.