Arh! That got me on track! It is a problem with the mirror. It detects a new version and downloads that, but overwrites the old file without truncating it first. Normally that is not a problem because the manifest file grows in size. But I recently removed a bunch of old release notes, so the new file is smaller, causing the downloaded file to contain junk at the end.
I have fixed the mirror server and now it serves the correct file.