Upgrade from 20.10 to 21.04 fails with “Authentication failed”

upgrading fails on my machine (same using the Software Updater) with:

goetz@Ubuntu-Mac:~$ sudo do-release-upgrade 
Checking for a new Ubuntu release
Get:1 Upgrade tool signature (819 B)                                                                                      
Get:2 Upgrade tool (1.276 kB)                                                                                             
Fetched 1.276 kB in 0s (0 B/s)                                                                                            
authenticate 'hirsute.tar.gz' against 'hirsute.tar.gz.gpg' 
Authentication failed
Authenticating the upgrade failed. There may be a problem with the network or with the server.

Does anyone have an idea how to solve this?

Please find below my sources.list:

goetz@Ubuntu-Mac:~$ cat /etc/apt/sources.list
# deb cdrom:(Ubuntu 20.04.2.0 LTS _Focal Fossa_ - Release amd64 (20210209.1))/ focal main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://de.archive.ubuntu.com/ubuntu/ groovy main restricted
# deb-src http://de.archive.ubuntu.com/ubuntu/ focal main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://de.archive.ubuntu.com/ubuntu/ groovy-updates main restricted
# deb-src http://de.archive.ubuntu.com/ubuntu/ focal-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://de.archive.ubuntu.com/ubuntu/ groovy universe
# deb-src http://de.archive.ubuntu.com/ubuntu/ focal universe
deb http://de.archive.ubuntu.com/ubuntu/ groovy-updates universe
# deb-src http://de.archive.ubuntu.com/ubuntu/ focal-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://de.archive.ubuntu.com/ubuntu/ groovy multiverse
# deb-src http://de.archive.ubuntu.com/ubuntu/ focal multiverse
deb http://de.archive.ubuntu.com/ubuntu/ groovy-updates multiverse
# deb-src http://de.archive.ubuntu.com/ubuntu/ focal-updates multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://de.archive.ubuntu.com/ubuntu/ groovy-backports main restricted universe multiverse
# deb-src http://de.archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu focal partner
# deb-src http://archive.canonical.com/ubuntu focal partner

deb http://security.ubuntu.com/ubuntu groovy-security main restricted
# deb-src http://security.ubuntu.com/ubuntu focal-security main restricted
deb http://security.ubuntu.com/ubuntu groovy-security universe
# deb-src http://security.ubuntu.com/ubuntu focal-security universe
deb http://security.ubuntu.com/ubuntu groovy-security multiverse
# deb-src http://security.ubuntu.com/ubuntu focal-security multiverse

# This system was installed using small removable media
# (e.g. netinst, live or single CD). The matching "deb cdrom"
# entries were disabled at the end of the installation process.
# For information about how to configure apt package sources,
# see the sources.list(5) manual.

-Best regards, ii7017-