ZFS fails with error 6 on upgrade to 11.1

[Sebastian Schwarz]

On 2017-07-27, Trond Endrestøl wrote:
> Off the top of my head I would ensure zfs.ko is loaded at boot time,
> by adding zfs_load="YES" to /boot/loader.conf within the
> ssd/ROOT/freebsd11.1.0 filesystem.

zfs_load="YES" is already present in loader.conf.

> Make sure the boot pool is in its imported state, and if you use
> /boot/zfs/zpool.cache at all, make sure it's up-to-date.

I tried both updating and disabling the cachefile, but in both
cases I still get the same error:

> Mounting from zfs:ssd/ROOT/freebsd11.1.0 failed with error 6.