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[For YouTube] Incorrect Thumbnail URLs being fetched sometimes.

See original GitHub issue

Checklist

I’m reporting a broken site support
I’ve verified that I’m running youtube-dl version 2021.05.16
I’ve checked that all provided URLs are alive and playable in a browser
I’ve checked that all URLs and arguments with special characters are properly quoted or escaped
I’ve searched the bugtracker for similar issues including closed ones

Verbose log

[debug] Command-line args: ['--cookies', 'cookies.txt', '--verbose', '--merge-output-format', 'mp4', '--write-thumbnail', '--embed-thumbnail', '--output', '[%(upload_date)s] %(title)s (%(id)s).%(ext)s', '--', 'https://www.youtube.com/watch?v=ACV-i-b10io']
[debug] Encodings: locale cp65001, fs mbcs, out cp65001, pref cp65001
[debug] youtube-dl version 2021.05.16
[debug] Python version 3.4.4 (CPython) - Windows-10-10.0.18362
[debug] exe versions: ffmpeg 2021-03-09-git-c35e456f54-full_build-www.gyan.dev, ffprobe 2021-03-09-git-c35e456f54-full_build-www.gyan.dev
[debug] Proxy map: {}
[debug] Default format spec: bestvideo+bestaudio/best
WARNING: Unable to download thumbnail "https://i.ytimg.com/vi/ACV-i-b10io/maxresdefault.jpg?v=6039625b": HTTP Error 404: Not Found

Description

While trying to download certain YouTube videos, Youtube-DL fetches the wrong URL and won’t download any thumbnail at all. In the example from the log above, the URL for the thumbnail is: http://i3.ytimg.com/vi/ACV-i-b10io/maxresdefault.jpg

It seems this only occurs with thumbnails on URLs beginning with i3.ytimg.com/vi/xxxxxxxxxxxxx