Our rancher-server crashed. We noticed this because our web interface was no longer available. The active nodes and the load balancers are still working, we can still access our various webapplications running in the various containers.
When we rebooted the server, docker did no longer manage to get back up. I have been debugging it for a couple of hours and finally discovered that after removing the folder for the faulty container from /var/lib/docker/containers/, docker came back up.
Our machine running rancher server has only 2 containers: 1 for rancher-data and one for rancher-server.