So, I think I solved it...
Stopped all containers, and started one at a time until I found that Rebuild-DNDC was looking like the bad guy. Obviously Rebuild-DNDC is the last one in my startup order too... so it took a while due to the 5 minutes wait time for each container 🤡
Rebuild-DNDC had cached all my *RR-containers for whatever reason, and tried a rebuild every 5 minutes triggered from the default configured CRON-job.
I was not using any VPN-network for the *RR-containers. Probably something I've tested for a long time ago.
Solution:
Stop Rebuild-DNDC
Delete all .tmp-files in Rebuild-DNDC /config-folder (in my case it was in "/mnt/user/appdata/rebuild-dndc"
Start up Rebuild-DNDC
Now it works like it should again.
What still makes me annoyed is that I don't understand why there is no logs in the containers that "Rebuild-DNDC" tries to rebuild, or in the docker.log? Should be traceable some way that a container schedules a stop/start for other containers.