Installation was done with foreman-install. foremann-ansible-core was not installed because of a file collision. /usr/bin/json_inventory.sh is in ruby-smart-proxy-ansible and ruby-foreman-ansible-core. Same isse as Can't attribute Ansible Roles to a host and Can't install ruby-foreman-ansible-core. After installing the package with the option Dpkg::Options::="–force-overwrite" and restarting the foreman-proxy its working.
