been getting some 404s as well when updating my machines even after updating my mirror list (US and Canada mirrors)
error: failed retrieving file 'sweet-theme-full-git-r308.6e2ef01-1-x86_64.pkg.tar.zst' from cdn-mirror.chaotic.cx : The requested URL returned error: 404
error: failed retrieving file 'protontricks-git-1.10.2.r1.g124fc7a-1-any.pkg.tar.zst' from cdn-mirror.chaotic.cx : The requested URL returned error: 404
updates seem to complete still somehow (at least after I've dealt with some recent ffmpeg related conflicts)
I'll have to look into commenting out mirrors as @meanruse mentioned as it seems to be an ongoing issue.