Just in case somebody else has the same problem. Created now a new Digital Ocean droplet with Ubuntu 20.04 and could get n8n up and running like this:
cd ~
curl -sL https://deb.nodesource.com/setup_12.x -o nodesource_setup.sh
sudo apt install nodejs -y
npm i -g n8n --unsafe
n8n
But just to say it again. Is not recommended. Should be installed with a different user, security should get activated, SSL, … . So still advise strongly to use the Server Setup Guide from above.