since a few days I cannot apt update any more, it says
Hit:1 http://repo.steampowered.com/steam precise InRelease
Hit:2 http://de.archive.ubuntu.com/ubuntu xenial InRelease
Hit:3 http://security.ubuntu.com/ubuntu xenial-security InRelease
Hit:4 http://ppa.launchpad.net/cybre/sni-qt-eplus/ubuntu xenial InRelease
Hit:5 http://de.archive.ubuntu.com/ubuntu xenial-updates InRelease
Hit:6 http://de.archive.ubuntu.com/ubuntu xenial-backports InRelease
Hit:7 http://ppa.launchpad.net/elementary-os/daily/ubuntu xenial InRelease
Get:8 http://packages.elementary.io/appcenter xenial InRelease [5.893 B]
Hit:9 http://ppa.launchpad.net/elementary-os/stable/ubuntu xenial InRelease
Hit:10 http://ppa.launchpad.net/graphics-drivers/ppa/ubuntu xenial InRelease
Hit:11 http://ppa.launchpad.net/jonathonf/vim/ubuntu xenial InRelease
Hit:12 http://ppa.launchpad.net/nextcloud-devs/client/ubuntu xenial InRelease
Hit:13 http://ppa.launchpad.net/ondrej/php/ubuntu xenial InRelease
Hit:14 http://ppa.launchpad.net/elementary-os/os-patches/ubuntu xenial InRelease
Hit:15 http://ppa.launchpad.net/teejee2008/ppa/ubuntu xenial InRelease
Hit:16 http://ppa.launchpad.net/ubuntu-mozilla-daily/firefox-aurora/ubuntu xenial InRelease
Err:8 http://packages.elementary.io/appcenter xenial InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A74F73EFFE70B91C
Reading package lists... Done
W: GPG error: http://packages.elementary.io/appcenter xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A74F73EFFE70B91C
E: The repository 'http://packages.elementary.io/appcenter xenial InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
Googling for this tells me I should somehow add the public key but solutions like
$ sudo LANG=C apt-key adv --keyserver keyserver.ubuntu.com --recv-keys A74F73EFFE70B91C
Executing: /tmp/tmp.TzN2XR5fIq/gpg.1.sh --keyserver
keyserver.ubuntu.com
--recv-keys
A74F73EFFE70B91C
gpg: requesting key FE70B91C from hkp server keyserver.ubuntu.com
gpgkeys: key A74F73EFFE70B91C not found on keyserver
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
gpg: keyserver communications error: key not found
gpg: keyserver communications error: bad public key
gpg: keyserver receive failed: bad public key
do not work as well. Maybe elementary uses another keyserver than ubuntu? Or something else is broken?
AppCenter also aborts with an error message (same reason)
I have read this question but (as above) the solutions provided here do not work in my case
Would be very glad if someone could help, thanks a lot in advance
