geli partition accepts passphrase but will not mount

Hello,

I'm running 11.0-CURRENT #0 r281867. I've followed the instructions
given at
https://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/disks-encrypting.html
section 18.12.2.

I was able to create the encrypted slice and mount it. I transferred
some documents to that drive for safekeeping and unmounted it. Updated
the machine and added these lines to the kernel:

options GEOM_ELI
device crypto

but before I could rebuild the system later, it (some hours after this)
went into a hard lock. Powering off then on again rebooted the system
and from there I was able to run buildworld and friends. Rebooted again,
now trying to mount the disk:

root_at_:~ # geli attach -k /root/da0p1.key /dev/da0p1
Enter passphrase:
root_at_:~

root_at_:~ geli status
     Name  Status  Components
da0p1.eli  ACTIVE  da0p1

Trying to mount it, this happens:

root_at_:~ # mount /dev/da0p1.eli /mnt
mount: /dev/da0p1.eli: Invalid argument

I think this needs fsck but I get
root_at_:~ # fsck -p -t ffs /dev/da0p1.eli
Cannot find file system superblock

What can I do?

thanks,
-- 
John