I got following error after updating the sources.list, including the owncloud repo:
W: GPG-Fehler: http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_16.04 Release: Die folgenden Signaturen konnten nicht überprüft werden, weil ihr öffentlicher Schlüssel nicht verfügbar ist: NO_PUBKEY 4ABE1AC7557BEFF9
W: The repository 'http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_16.04 Release' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
Does anyone have had experiences with that issue?