I carried out the installation exactly according to the following HowTo: Ubuntu + Docker + Nextcloud + Talk + Collabora - 
How to - Nextcloud community. This HowTo is using docker-compose and therefore a docker-compose.yml, which has some variables in it. I made the mistake that I have assigned the localhost IP to the variable NEXTCLOUD_IPADDRESS. Thus some port bindings was only set to the localhost ip and external access was not possible.