$ sudo do-release-upgrade
Checking for a new Ubuntu release
Hole:1 Upgrade tool signature [198 B]
Hole:2 Upgrade tool [1.230 kB]
Es wurden 1.230 kB in 0 s geholt (0 B/s)
authenticate 'wily.tar.gz' against 'wily.tar.gz.gpg'
gpg exited 1
Debug information:


gpg: Unterschrift vom Do 04 Feb 2016 19:02:35 CET mittels DSA-Schlüssel ID 437D05B5
gpg: /tmp/ubuntu-release-upgrader-uuwcsvdv/trustdb.gpg: trust-db erzeugt
gpg: FALSCHE Unterschrift von »Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>«

Authentication failed
Authenticating the upgrade failed. There may be a problem with the network or with the server.