Hi @LudoNew and welcome!
I'm seeing the above error which is definitely related to add_k8s_metedata processor. However it is for another container and not for the one you are mentioning. Do you see other similar errors in the output?
Also you might want to try with autodiscover instead which by default adds k8s+docker metadata.
An example config can be found at https://github.com/elastic/beats/blob/master/deploy/kubernetes/filebeat-kubernetes.yaml#L22-L31
Let me know about any updates!