Ubuntu verweigert do-release-upgrade

Ubuntu stellt sich etwas bockbeinig beim do-release-upgrade an.

Fehler:

Invalid package information

After updating your package information, the essential package
ubuntu-minimal could not be located. This may be because you have
no official mirrors listed in your software sources, or because of
excessive load on the mirror you are using. See /etc/apt/sources.list
for the current list of configured software sources.
In the case of an overloaded mirror, you may want to try the upgrade
again later.

Unteranderem erfolgt die Meldung etwas kryptischer.

Lösung:

do-release-upgrade --allow-third-party