Below are two example mirrors causing the problem. The direct link shows 404 not found. Browsing up to the "jessie" directory confirms that the "updates" directory does not exist, or is not publicly accessible. Same result via web proxy. Tried
httpredir.debian.org and
cdn.debian.net. It seems that various mirrors have this problem. It does not appear to be related to apt or the redirector.
Does this mean that these mirrors are incomplete? If so, should they be auto removed? Failing to fetch updates seems like a major security hole.. Could the redirector or aptitude maybe auto try another mirror when one responds 404?