Error while enrolling: empty access_token

I have successfully installed a full ELK Docker stack including Filebeat. When I want to enroll a Filebeat instance, I get the following error:

Error while enrolling: empty access_token

According to the source, the error can be anything, if the response is not nice.

filebeat.yml:

filebeat.config:
    modules:
        path: /usr/share/filebeat/config/modules.d/*.yml
        reload.enabled: false

filebeat.modules:
    -   module: apache2

processors:
    -   add_cloud_metadata: ~

output.elasticsearch:
    hosts: 'http://elasticsearch:9200'
    username: 'elastic'
    password: 'changeme'

setup.kibana:
    host: "http://kibana:5601/"
    username: "elastic"
    password: "changeme"

name: "apache-test"

What is wrong?