Thanks, thought they only confirmed what xenolevis and I suspected: You had no config.json file and the starting container created it as directory, from there on nothing works. You need to remove the directory, create an empty json file and remove and restart the watchtower container by running the following commands “for you” this time:
cd /etc/egroupware-docker
docker-compose stop watchtower
docker-compose rm -f watchtower
rm -rf /root/.docker/config.json
echo "{}" > /root/.docker/config.json
docker-compose up -d
Ralf