Hello.

In case you're using our upstream repositories for Debian or Ubuntu, as suggested on https://www.knot-resolver.cz/download/
you'll be running into their signing key expiring since today.  As we didn't update it in time, you'll have to update it manually by re-running:

wget https://secure.nic.cz/files/knot-resolver/knot-resolver-release.deb
dpkg -i knot-resolver-release.deb

Ticket: https://gitlab.nic.cz/knot/knot-resolver/-/issues/747

We also forgot to add Ubuntu 22.04, so that is fixed now, too.

--Vladimir