Background image not shown

Hi there, first great plugin:-)
We have only the problem that background image is not shown but is already set in the mega menu element with a image https://bennitrymsec2021.mediadesign-linke.de/wp-admin/upload.php?item=1802

Do you have a idea whta this could be about / maybe beacause we use a child theme?

Thanks and best regards

The page I need help with: [log in to see the link]