Problem upragding to focal fossa 20.04 authenticate 'focal.tar.gz' against 'focal.tar.gz.gpg' Authentication failed

I tried to upgrade from 18.04 to 20.04 and got the same error.

The problem was that the 'bionic-updates' channel wasn't active in my /etc/sources.list I had only 'bionic' and 'bionic-security'.

After adding a line to the sources.list:

deb http://hu.archive.ubuntu.com/ubuntu/ bionic-updates main restricted

sudo apt update && sudo apt dist-upgrade it installed about 300+ packages and after that do-release-upgrade worked.