Error updating zorin 16 pro system

After starting sudo apt-get update some actions worked but then an error message occours:
E: Das Depot »http://ppa.launchpad.net/menulibre-dev/daily/ubuntu focal Release« enthält keine Release-Datei.
N: Eine Aktualisierung von solch einem Depot kann nicht auf eine sichere Art durchgeführt werden, daher ist es standardmäßig deaktiviert.
N: Weitere Details zur Erzeugung von Paketdepots sowie zu deren Benutzerkonfiguration finden Sie in der Handbuchseite apt-secure(8).

Translation:
E: The "http://ppa.launchpad.net/menulibre-dev/daily/ubuntu focal Release" depot does not contain a release file.
N: Updating from such a repository cannot be done in a secure way, so it is disabled by default.
N: For more details about creating package repositories and their user configuration, see the apt-secure(8) manual page.

Whats going wrong?

That is a development repository. I would suggest removing it. The app Menulibre is already contained in the Universe Repository.
You can do so in terminal

sudo add-apt-repository --remove ppa:launchpad.net/menulibre-dev/daily/ubuntu

Or open Software & Updates from your app menu and select the Other Software tab. Select launchpad.net/menulibre-dev/daily/ubuntu on the list and then click the remove button on the bottom toolbar.
Once done, in terminal:

sudo apt update && sudo apt upgrade

1 Like

I could delete the repository. But after that a new error occoured:
pk-client-error-quark: E:http://ch.archive.ubuntu.com/ubuntu focal-updates inRelease ist not available

Do you have the feeling we are about to fall down a rabbit hole?

I think we may go through all repos and find the servers are being unreachable...

Can you please open Software & Updates from your app menu and on the very first tab, Download from - Set to Main Server.
Do not use your local or regional server; it appears to be having server outages.