Okay, had some 2.1 version installed.
I used sudo apt-get purge owncloud-client to remove it.
then I added the Repo to my apt-list.
I tried to update but had some broken dependencies. I used sudo apt autoremove and sudo apt-get install -f
After that I was able to update again.

Now I am back at the start:
sudo apt-get update
OK:1 http://security.ubuntu.com/ubuntu xenial-security InRelease
OK:2 http://ppa.launchpad.net/openshot.developers/ppa/ubuntu xenial InRelease
OK:3 http://de.archive.ubuntu.com/ubuntu xenial InRelease
OK:4 http://de.archive.ubuntu.com/ubuntu xenial-updates InRelease
OK:5 http://de.archive.ubuntu.com/ubuntu xenial-backports InRelease
Paketlisten werden gelesen... Fertig
$ sudo sh -c "echo 'deb http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_16.04/ /' > /etc/apt/sources.list.d/owncloud-client.list"
$ sudo apt-get update
OK:1 http://security.ubuntu.com/ubuntu xenial-security InRelease
OK:2 http://de.archive.ubuntu.com/ubuntu xenial InRelease
OK:3 http://ppa.launchpad.net/openshot.developers/ppa/ubuntu xenial InRelease
Ign:4 http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_16.04  InRelease
Holen:5 http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_16.04  Release [1.025 B]
OK:6 http://de.archive.ubuntu.com/ubuntu xenial-updates InRelease
OK:7 http://de.archive.ubuntu.com/ubuntu xenial-backports InRelease
Holen:8 http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_16.04  Release.gpg [481 B]
Ign:8 http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_16.04  Release.gpg
Holen:9 http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_16.04  Packages [31,0 kB]
Es wurden 32,5 kB in 0 s geholt (32,9 kB/s).
Paketlisten werden gelesen... Fertig
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.