ZFS mount fails after upgrade to 11.0

[Erwan Legrand]

I am upgrading a FreeBSD 10.3 AMD64 system with a ZFS root to 11.0
with freebsd-update. After the first reboot, mounting root fails with
the following error:

"""
Solaris: NOTICE: cannot find the pool label for 'tank'
Mounting from zfs:tank/root failed with error 5.
"""

Moreover, I can't type anything in the "mountroot>" prompt. My USB
keyboard does not appear to be recognized at this point, although it
worked with the boot loaded.

If I select the old kernel in the boot loader, the root filesystem is
mounted successfully and the system boot as usual.

Any suggestions regarding how to debug/fix this?

Thanks,

Erwan Legrand