Hi, @cowperwood !
Thanks for your post. This is a pretty common domain error. When you visit a page that is not SSL secured but still using https in the URL, most browsers will view this as a security issue. When you add your custom domain, a new SSL certificate is automatically created. This process can take up to 48 hours but it sounds like we're already past that point. If that's the case, then I'd recommend checking the following:
You can also learn more about domains and SSL statuses here. If you're still unsure, let me know and we can take a closer look.
All the best!