This morning there have been problems with the Release file for Squeeze backports. The main problem being that it is automatically updated when a new package is added, but there haven’t been any new packages lately, so the Release file expired. They’re working on fixing the problem. In the meantime, we’re happy we got both our own repository and use approx almost everywhere.
Fixing it is as easy as editing the Release file for squeeze-backports in your approx cache (in our setup, it’s in /var/cache/approx/backports/dists/squeeze-backports/Release) and increase the Valid-Until date. Then resign the Release file with the key you use for your repository (mostly as a convenience, since all machines will have that key in their apt config) and renaming the resulting detached signature to Release.gpg. Now approx will serve a valid Release file, signed by a key it knows.
If you don’t have your own repository, you can sign the file with your own gpg key and import your public key into apt with the following command:
sudo apt-key adv --keyserver your.default.keyserver --recv-key 12345678 (with 12345678 being your key ID of course)
Once the squeeze-backports repository is fixed, approx will notice a newer Modified-Since header and overwrite the files again.
Update 11:32 They fixed the squeeze-backports repositories.