Already spent several days with my hosting provider. They can’t seem to find a reason for this either.
So the original code to make the background transparent up at the top didn’t work so I tried this instead:
.x-container.main:not(.x-row):not(.x-grid)::before {
background-color: transparent;
}
Now it works. Of course, to figure that out I had to use the “inspect element” inside the “theme options/layout and design” screen where the background image is uploaded.