I use Version 1.34.1 and the wiki says An absolute url to the image to use for the icon, this is recommended but not required, but absolute does not mean an foreign/remote url here. It works for me only, if I have this icon on the server. I tried to use a remote one, because my hoster offers some icons (for sponsoring).
Topic on Manual talk:$wgFooterIcons/Flow
What error do you get? What's the generated URL of the icon? Maybe the external host doesn't allow hotlinking.
The "error" is, it doesn't show anything. My hoster offers hotlinking explicit via c&p source code for including. What do you mean with 'generated URL' ?
You'll have to inspect the HTML where the icon is supposed to be present (but it doesn't display for you) and see what's the URL it points to, or if there's no icon URL at all.... Open that URL on your browser and see if the image gets displayed or you see an error instead...
The img-URL is accessable direcly in the browser. If I use the FF-tool 'analyse item' and I hover the 'img scr' it says 'Couldn't load graphic'. So I can't find any concrete problem.
You can look at the network tab of the developer console and see what's the server response for that particular URL when requested from the page.
Thanks for the decisive hint! It doesn't show the icon due to my AdBlocker!