Comment 1 for bug 771729

I got the same error when I was connected to a network that requires authentication on MAC basis to get through the proxy. It seems apt isn't validating anything or at least isn't refusing the lists and keeping them anyway. The package lists were corrupted by the response of the network proxy. And for lists downloaded prevously, the update-manager does not check the descriptions it is retrieving from the proxy.

Workaround in my case: turned off the automatic check for updates.

This is really annoying for people often using public network access or a company network.