Comment 4 for bug 1206824

I just wanted to add a comment, for any others who come to this via Google.

I also hit this exact same error, but it turned out not to be caused directly by the network proxy. I had just run apt-add-repository, and that had failed to import the GPG key:

root@buildhost:/home/mblokzij/ws-qvpc# add-apt-repository ppa:x2go/stable
 Quick howto to turn your machine into an X2Go server:
...
gpg: keyring `/tmp/tmpf3fzey4y/secring.gpg' created
gpg: keyring `/tmp/tmpf3fzey4y/pubring.gpg' created
gpg: requesting key 0A53F9FD from hkp server keyserver.ubuntu.com
gpgkeys: key A7D8D681B1C07FE41499323D7CDE3A860A53F9FD can't be retrieved
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0

Note the last line.

After manually importing the key, apt-get update ran fine. I used:

apt-key adv --recv-keys --keyserver keys.gnupg.net 0A53F9FD

I realise I used a different keyserver, but I copy/pasted the key id from the logs above.