Thanks for fixing your HTTPS setup. I could clearly see the issue and the cause now. I've also realized that the hint is in the title of this thread. Sorry about that.
What you could do since you're not using Cornerstone is to create a Global Block with a Content Modal Element. Then, use the global block's shortcode and add it to your content. To clearly see what I mean, please watch the screencast at https://youtu.be/OrbY6q1WjS8
Regarding updating X and Cornerstone, thankfully, this issue is not related to updates. It is still recommended though. You might encounter another issue in the future and updating might be really necessary especially for compatibility for major updates of WordPress and other plugins.