Comment 16 for bug 127263

That's really not sufficient. If If-Modified-Since is going to be used at all, the three different
http://changelogs.ubuntu.com/meta-release* files need to be cached as three separate files.
(Though I'm not sure how ~/.update-manager/meta-release plays into it.)

A workaround:

sudo touch -t 197001010000 /var/lib/update-manager/meta-release

before running update-manager -d.