Hi Piotr,
Seems to be Ubuntu-related issue. I've found a possible resolution:
Try disabling the apt-cacher service and comment anything you might have configured incode:/etc/apt/apt.conf.d/90-apt-proxy.conf
src: https://askubuntu.com/questions/811370/14-04-5-to-16-04-update-failed-xenial-tar-gz-gpg-missing
Let us know if that helped.
Perhaps this service might be used in case of EAC/EAA upgrade (development would have to comment on this) but then you should simply to re-enable the service after an upgrade if disabling it does its job as a workaround.
More about apt-cacher: http://manpages.ubuntu.com/manpages/cosmic/man8/apt-cacher.8.html
Kind regards,
Tomasz