Can't upload header image

I’m using the Hemingway theme, downloaded from here: https://www.andersnoren.se/teman/hemingway-wordpress-theme/

I can’t see my uploaded custom header images in the “media library” (The library that is found in customize> header image>(choose image >) media library)

It stopped working when I activated my child theme. When I switch back to the original theme my custom header is visible and I can see all the images I’ve tried to upload (which obviousy was possible to do, despite the error code saying they didn’t load) but cannot see them in the library when the child theme is activated.

When I try to upload a new custom header I get the error code “An error occurred in the upload. Please try again later.”

My conclusion is that something in the child theme is messing with the visibility of the uploading process. However, the pics are not only invisible, I also cannot change the header image when child theme is activated.

I’m new to child themes, so I’m not sure how to troubleshoot. Please help!