Why am I getting "Authentication failed" while running do-release-upgrade from 19.10 to 20.04 LTS with configured squid-deb-proxy client?

I’m running squid-deb-proxy server in my local network. It works well for all package downloads while running all APT commands. It is configured for all possible known URLs of the mirrors including old-releases. The Ubuntu 19.10 client has installed client side – the squid-deb-proxy-client package.

$ dpkg -l squid-deb-proxy-client | tail -n1
ii  squid-deb-proxy-client 0.8.14+nmu2  all          automatic proxy discovery for APT based on Avahi

$ sudo apt update 
Hit:1 http://archive.ubuntu.com/ubuntu eoan InRelease
Hit:2 http://archive.ubuntu.com/ubuntu eoan-updates InRelease
Hit:3 http://archive.ubuntu.com/ubuntu eoan-backports InRelease
Hit:4 http://security.ubuntu.com/ubuntu eoan-security InRelease
Reading package lists... Done
Building dependency tree       
Reading state information... Done
All packages are up to date.

But when I try to start upgrade process to get 20.04 LTS – using sudo do-release-upgrade in the terminal I got the following error message:

$ sudo do-release-upgrade 
Checking for a new Ubuntu release
Your Ubuntu release is not supported anymore.
For upgrade information, please visit:
http://www.ubuntu.com/releaseendoflife

Get:1 Upgrade tool signature [1 554 B]                                                                                                              
Get:2 Upgrade tool [1 342 kB]                                                                                                                       
Fetched 1 343 kB in 0s (0 B/s)                                                                                                                      
authenticate 'focal.tar.gz' against 'focal.tar.gz.gpg' 
Authentication failed
Authenticating the upgrade failed. There may be a problem with the network or with the server. 

I do not see anything special in the logs on squid-deb-proxy server side:

==> /var/log/squid-deb-proxy/access.log <==
1595069772.862    212 192.168.3.47 TCP_MISS/200 3349 CONNECT changelogs.ubuntu.com:443 - HIER_DIRECT/91.189.95.15 -
1595069774.726      2 192.168.3.47 TCP_MEM_HIT/200 1950 GET http://archive.ubuntu.com/ubuntu/dists/focal-updates/main/dist-upgrader-all/current/focal.tar.gz.gpg - HIER_NONE/- application/x-gzip
1595069775.410    681 192.168.3.47 TCP_HIT/200 1342082 GET http://archive.ubuntu.com/ubuntu/dists/focal-updates/main/dist-upgrader-all/current/focal.tar.gz - HIER_NONE/- application/x-gzip

How should I fix the problem with Authentication failed to run the upgrade through proxy server normally? I do not want to switch proxy server (or exclude its configuration from the client) off to fix the problem.

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Notes:

1. For possible close-voters: the same happening when I try to upgrade 18.04 LTS to 20.04 LTS with sudo do-release-upgrade -d.
2. I have reported bug 1888058 to launchpad.
3. Removing squid-deb-proxy-client deb-package and specifying proxy by Acquire::http::Proxy "http://192.168.12.34:8000"; in /etc/apt/apt.conf does not change anything.

cd /tmp
http_proxy=http://192.168.12.34:8000 wget --no-cache -c http://archive.ubuntu.com/ubuntu/dists/focal-updates/main/dist-upgrader-all/current/focal.tar.gz
http_proxy=http://192.168.12.34:8000 wget -c http://archive.ubuntu.com/ubuntu/dists/focal-updates/main/dist-upgrader-all/current/focal.tar.gz

sudo service squid3 restart
sudo service squid-deb-proxy restart

/etc/apt/sources.list

deb http://in.archive.ubuntu.com/ubuntu/ focal main restricted universe multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ focal main restricted universe multiverse

deb http://in.archive.ubuntu.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ focal-updates main restricted universe multiverse

deb http://in.archive.ubuntu.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ focal-security main restricted universe multiverse

deb http://in.archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse

deb http://archive.canonical.com/ubuntu focal partner
deb-src http://archive.canonical.com/ubuntu focal partner

sudo apt-get install --reinstall ubuntu-keyring