Upgrade to 5:18.09.0~3-0~debian-stretch: all images are gone :-(

After the upgrade to version 5:18.09.0~3-0~debian-stretch all images and configured containers are gone. /etc/docker/daemon.json was not touched.

{
    "data-root": "/export/docker-data",
    "storage-driver": "overlay2",
    "bip": "10.20.0.1/16",
    "userns-remap": "default"
}

How can this happen? /export/docker-data still contains GBytes of files. ???

Every helpful comment is highly appreciated.

Harri