The key is not in there. Executed your suggestion but grep returns nothing. So that must be the problem. But: now I found the way to add the key in there with the following
sudo gpg --keyserver hkp://keyserver.ubuntu.com --no-default-keyring --keyring /usr/share/keyrings/ubuntu-archive-keyring.gpg --recv-keys 871920D1991BC93C
and now it works! 
Gerhard, thanks for your time and help