Yeah I can see you’re using SSL on your /customize/ page. That’s most likely the issue.
Our next plugin release next week will have SSL support for HTML5 as well, but right now your pre-release is writing the iframe URL with http://.
Until then- temporary solutions:
1) Can you exclude that page from SSL so it’s regular http:// instead of https:// ?
I think there are some plugins that let you manage SSL on some pages and not others if you’re not able to do it via PHP or .htaccess yourself
2) Edit some php… (hacky)
If you are able to change the line that writes the iframe URL and replace http:// with https:// it would work.
3) Turn off full-site SSL and just use SSL for checkout (woocommerce setting)