In the ByWater Solutions coverflow (v2.4.42) on Koha 20.11.02, the custom URL option, which sets a default image if no image is found, in the coverflow plugins configuration no longer works. In my instance, the value set there is currently https://library.craigbutosi.ca.NoImage.jpg, which is accessible via any browser. The file above is stored in /usr/share/koha/opac/htdocs. It's as if that configuration field is not being read. Rather than the custom image I have used in previous versions of Koha and the coverflow (https://library.craigbutosi.ca/NoImage.jpg), what displays is the fallback image that ByWater Solutions uses (the src attribute in the CSS reads: "https://images-na.ssl-images-amazon.com/images/P/0391007742.01.LZZZZZZZ.jpg", which seems unexpected if the custom URL field is set.
You'll want to open issue for the Coverflow plugin on its Github page: https://github.com/bywatersolutions/koha-plugin-coverflow/issues