Safari Security: [Warning] [blocked] The page at https://url.com/ was not allowed to run insecure content from http://url.us5.list-manage.com...

[QuintinS]

This seems to be an issue exclusive to Safari and Safari mobile.

It is related to how the Ajax request URL is built:

settings.url.replace('/post?', '/post-json?').concat('&c=?')

Should be:

settings.url.replace('/post?', '/post-json/?').concat('&c=?')

Having the ? without a trailing / causes a redirect to and http:// URL, which causes Safari to throw an error.