I have the exact same issue.with my RasPi4 and OMV5 when installing the docker image "5:19.03.13~3-0~debian-buster".
During the installation at some point I receive the following error:
In the OMV-Extras I set the IP tables to legacy.
lsmod | grep table
delivers this result
sudo modprobe iptable_nat
When clicking on restart in OMV, I receive the following error message again and again:
Docker storage is /var/lib/docker.
The folder permissions look a bit restrictive. Could that be the problem?
Any other ideas?