I had this issue just now but I don't have a cache and I ran a backup of the Flash drive and then reboot the Unraid server and once it came back up I didnt have the issue and I was able to add new docker containers and start all of my containers without issue. I will keep and eye on my server to see if it pops back up.
If any one knows what the issue really is then I would like to know so that I can implement a proper fix.