Sadly this is not the real problem.
All these errors happen at 4:00 when I shutdown Mattermost to backup the DB.
The Mattermost i/o websocket timeouts appear at different times and then there are no corresponding errors logged for nginx.
I posted my nginx configuration to support but their answer was “So in this case, unless you are having an issue in your Mattermost, otherwise you can actually ignore this error message.”
Basically it says: ignore error logs in your Mattermost logfile which is not what I expected. Why are they then logged at all?