Thank you for the answer and the reference to personal responsibility here in the forum :-) This is very relevant, and my post was not very clever. Here is why:
As I am relatively new to docker (sic!), specially in a complex setup with mailcow, there is still some uncertainty on the subject of Docker. I am not always sure what is what.
I saw the thread about containerd already, but was unsure if it is really related to the observation.
I checked meanwhile which container is causing the error message. It is
mailcow-dockerized_dockerapi-mailcow_2. All other containers do not cause any error message in the journal by
docker stats <containerid>.
What do you think? Is it still related to docker/containerd? Or is it something with the mailcow setup after all?
Docker version 20.10.14, build a224086