As discussed in the chat, your Apache configurator has
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*[^/])$ /$1/ [L,R=301,NE]
What it does is that if the file does not exist, it is checked with the addition /at the end. This would mean that the png file you really do not exist and should give you 404, but these rules give you 301 with the application /and then exit as 404.
So the problem is that the image is simply missing
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