<h2><code>error: failed retrieving file 'X' from Y : The requested URL returned error: 404</code></h2>
<br>
<p>
Replace <code>/etc/pacman.d/mirrorlist</code> with a new file generated at <ahref='https://archlinux.org/mirrorlist/'>https://archlinux.org/mirrorlist/</a>.
<br>
Then run <code>sudo pacman -Syyu</code> to update the package database with the new mirrorlist. Make sure to use 2 y's, to force the new changes.
</p>
<h2>Explanation</h2>
<p>
Your mirrorlist is outdated, possibly due to the recent <ahref='https://archlinux.org/news/git-migration-completed/'>pacman git migration</a>.