have you enabled the pdf.js viewer plugin? This looks like the browser wants to load the images for those buttons but is unable to reach them.
Please take a look in your server’s error log and see whether there are any errors related to accessing image files. These should be located in your OJS install at plugins/generic/pdfJsViewer/pdf.js/web/images.
In case you are not using the pdf.js plugin, this could point to a problem with your browser software or addons, because in this case, a local viewer software would be used.