I tried with a new ssh key (ssh-keygen) and used that (thanks Leo for the suggestion), I now get a slightly different error:
TASK [configure-buildfarm : Configure Kubernetes plugin] **********************************************************************************************************************************************************
fatal: [ansible_connection=local]: FAILED! => {"changed": true, "cmd": "java -jar /tmp/jenkins-cli.jar -remoting -s
http://jenkins-digger.127.0.0.1.nip.io groovy /tmp/kubernetes-plugin-config.groovy", "delta": "0:00:02.215772", "end": "2017-09-27 09:42:31.347839", "failed": true, "failed_when_result": true, "rc": 6, "start": "2017-09-27 09:42:29.132067", "stderr": "\nERROR: anonymous is missing the Overall/Read permission", "stderr_lines": ["", "ERROR: anonymous is missing the Overall/Read permission"], "stdout": "", "stdout_lines": []}
fatal: [127.0.0.1]: FAILED! => {"changed": true, "cmd": "java -jar /tmp/jenkins-cli.jar -remoting -s
http://jenkins-digger.127.0.0.1.nip.io groovy /tmp/kubernetes-plugin-config.groovy", "delta": "0:00:02.277280", "end": "2017-09-27 09:42:31.429172", "failed": true, "failed_when_result": true, "rc": 6, "start": "2017-09-27 09:42:29.151892", "stderr": "\nERROR: anonymous is missing the Overall/Read permission", "stderr_lines": ["", "ERROR: anonymous is missing the Overall/Read permission"], "stdout": "", "stdout_lines": []}
to retry, use: --limit @/home/Matthias/Work/Mobile.NEXT/digger-installer/sample-build-playbook.retry