The omv-plugins repo no longer exists. You need to remove any reference to it in /etc/apt/sources.list and any file that references it in /etc/apt/sources.list.d.
I guess i missunderstood your advise. I removed everything in in /etc/apt/sources.list and /etc/apt/sources.list.d.
apt-get update or upgrade does not bring an error now. on the other hand, nothing is updated anymore.
Can you please advise how to fix?