Screenshots not being uploaded in Media Manager

I am trying to upload screenshots for our first build validation in the "Prepare for Submission" page, but the image disappears after I drag it in the uploader. When I refresh the page there is a gray placeholder with a cloud icon. When checking the console after dragging an image to the uploader, I can see a "Upload operations error" error and a "[UPLOAD FAIL]" error, but there's no additional info.