Slider covers the entire page in theme 2017

When I put the slider on the homepage of 2017 theme (using the header-image.php modification), the image covers the entire page except for the menu. The text below, all the way to the footer, was covered. The slider size is set to the 2017 recommended size (2000×1200). Inspecting the elements (and modifying the corresponding css) didn’t change anything.
I’m not sure if this could be a z-index problem? But it almost looks like the slider is shown continuously twice on the page. Oh, and on mobile it’s completely blank.

Any help appreciated.
Thanks!

I didn't find the right solution from the Internet.

reference:
wordpress.org/support/topic/65328
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