What is the "mainpool" you have? Is this a ZFS mount? Admittedly I don't really follow the ZFS threads, but it really should be mounted outside of /mnt (ideally it should go into /mnt/disks/...)
Mounting a zfs pool within /mnt might explain some of the other weird errors that are appearing
For the actual problem at hand, several users have reported that when they put the docker.img onto a zfs pool issues result. The fix in that case would be to instead of an image you use a docker folder (settings - docker) instead. This change will result in you having to re-install the apps via Previous Apps (5 second process)