apt-get update reports 406 Not Acceptable

I recently noticed that apt-get update and aptitude update report some errors:

W: Failed to fetch http://ift.tt/1vPRjWy: Bad header line
W: Failed to fetch http://ift.tt/1rJyNTj: 406  Not Acceptable
W: Failed to fetch http://ift.tt/1vPRkcQ: 406  Not Acceptable
W: Failed to fetch http://ift.tt/1rJyO9z: 406  Not Acceptable
W: Failed to fetch http://ift.tt/1vPRiBH: 406  Not Acceptable
E: Some index files failed to download. They have been ignored, or old ones used instead.
E: Couldn't rebuild package cache

I looked at the server and found out that only Packages.gz and Packages.bz2, but not Packages exist. This also happens on the main server, not only on ftp.informatik.uni-frankfurt.de.

After some research I found out that all mirors recently updated to only contain the Packages file in an compressed format but no longer plain. How can I tell apt-get and/or aptitude to use the compressed files instead the plain ones?

I'm using Debian Wheezy.