Release upgrade ends up with error "Error during update

A problem occurred during the update. This is usually some sort of
network problem, please check your network connection and retry.

The server may be overloaded


Restoring original system state"
when doing do-release-upgrade -d to upgrade from karmik to lucid.
i using local mirror ftp://ubuntu.snacho.ru (also have http that works but not browseable)
when i change lines in /etc/apt/sources.list from local mirror to official http://archive.ubuntu.com/ubuntu/ all works fine.

I don't want to download 1Gb from internet (becouse of traffic cost). What is wrong with local mirror ? I can communicate with its owner, but what he needs to change on the mirror ?

p.s. he mirrors from mirror.yandex.ru/ubuntu