Same issue here after upgrading to 6.12.2 and creating a zfs pool for cache. Unknown if those two are related but based on similar posts suggesting problems with the OS I am now on 6.12.4.
For me this is definitely a docker img issue which can be traced to a container error.
The problem starts with a slow or unresponsive Unraid web Ui. I was able to isolate this to a problem with binhex-Delugevpn container becoming unresponsive that develops after days or weeks with no issues. Deluge web Ui never fully loads but all other container's work but are slow to load.
Running "docker stop binhex-Delugevpn" gets the Unraid UI working immediately and all other containers/services/vms are normal. However, Deluge does not actually stop, will not restart, nor force update. Running docker stop again... returns "docker is not running" (or something of that nature).
Stopping the docker service from the settings does stop the service and remove the Docker tab from the Unraid UI but, restarting gives error "docker service failed to start."
Now any attempt to stop the array puts it in the loop:
/usr/sbin/zpool export cache
cannot unmount '/mnt/cache/system': pool or dataset is busy
Commands ran to stop the loop
umount /var/lib/docker – returns “command not found”
umount /mnt/cache/system/libvirt/libvirt.img – returns “not mounted”
umount /usr/sbin/zpool – returns “not mounted”
umount -l /dev/loop2 – returns “not mounted”
umount /dev/loop3 - completes but does not stop the loop
shutdown -r now - finally kills the loop after a few minutes but starts parity check on restart for unclean shutdown.
I need to capture the container log from the terminal next time to figure out why Deluge becomes unresponsive. The array will cleanly stop with VMs and containers running prior to this container error. I have followed Spaceinvaders guide for docker repair, for both the container and the complete docker img rebuild, but still the problem continues.
For what it’s worth, I am using an unassigned HDD for deluge downloads and they are transferred directly to the array after completion, they do not hit the cache drive. Unraid is not reporting any disk errors on any drives.