Seems that image at https://commons.wikimedia.org/wiki/File:Toolbaricon_definition_list.png got corrupted. It is being used in some gadgets. I don't see any edits in past years, so I'm not sure, what could have brake it.
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Error details:
error: could not load image from https://upload.wikimedia.org/wikipedia/commons/a/a8/Toolbaricon_definition_list.png
URL: https://commons.wikimedia.org/wiki/File:Toolbaricon_definition_list.png#/media/File:Toolbaricon_definition_list.png
user agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:50.0) Gecko/20100101 Firefox/50.0
screen size: 1536x864
canvas size: 1519x679
image size: 22x22
thumbnail size: CSS: 679x679, screen width: 848.75, real width: 1024