Is the "insecure endpoint" on another installation?
No, it is not.
Are you loading any images on this page?
The only images being loaded are the product images, which show on the main page, as well as the company logo. Both of these, I would imagine, would be allowed for in AbanteCart.
I replaced the init.php with the uploaded version posted by Abolabo but that made things even worse. With the newer version of init.php the cart and payment pages wouldn't open properly. Once I switched back to my original init.php they opened normally.
Then things got even weirder. The page worked perfectly for approximately two hours the other day (security and everything else) but right after that the same security issue popped up again and additionally now the product images stopped showing on the main page (I did nothing different).
Actually, the point is moot now as I was spending way too much time on something that should work with no issues. Although I would like to stay with AbanteCart, I must remain with Zen Cart for now. I have no security issues with it and everything works as it should, although I like the AbanteCart interface much better (as well as the HTML receipts and notices).
Wish I could stay with AbanteCart but too many things broken in it with regard to standard items such as SSL certificates and product images.