looked at that error I was having with that url in browser tools and seems it was browser security or tracking prevention (edge chromium on Win10), and it was just coincidental that when I tried the http one on the browser address bar it wasn’t showing its content (now shows)
I had located it in some places in our wordpress installation and changed everywhere to https, but tracking prevention still grabs it anyway
however it’s better to use https:// to get such resources (even from inside https://) pages and most people suggest using that instead of // and instead of extracting and prepending the main page’s url protocol
unless one also wants to support archaic clients that may not support https at all
