I just noticed something: I have installed Sydney Child theme so that I can change the footer, but I am having trouble with doing that. When I added the footer.php file, the site preview completely stopped loading, just stayed on the bouncing dots for like 10 minutes. So I figured something with the Child theme caused the problem, so I deactivated and deleted it, and put back the Sydney main theme. When it has that, background image does scroll with the page.
However, I just installed the child theme again - with no modifications or anything in it yet, just the .zip file that I downloaded from you guys - and now the background image doesn’t scroll anymore.
If I activate the main Sydney theme again then it scrolls; if I activate the child theme again it doesn’t scroll.