Backports version contains more fixes than just the one listed in the bug above. If you just use the precise ordinary version (with the fix above), you get the following error
403 Forbidden file type or location
Because the first thing Centos does is check for local mirror locations with this
And something in that URL is not liked by apt-cacher-ng.
The precise backports version fixes this issue.
You can verify you have (or have fixed) the issue from your Ubuntu machine with
Set wget to use your apt-cacher-ng proxy
Tell wget to get the same file, but note the extra \'s to escape the '&' to get the full url to wget
wget --output-document=- http://mirrorlist.centos.org/?release=6\&arch=x86_64\&repo=os
Answers back with 403 error as above, or correct with list of mirrors...