Mounting some Dataset Fails

Binary package hint: zfs-fuse

I'm not able to mount some zfs datasets.
I upgraded from 0.5.1 to version 0.6.0 which will be delivered with lucid.
Some Datasets get mounted but if i try to mount zwo of the sets, which did not get automatically mounted i get the error:
zfs: lib/libavl/avl.c:637: avl_add: Assertion `0' failed.

I could not find anything about this on the web.

And it failed already before i started the scrub

root@utgard-2:/# zpool status tank
  pool: tank
 state: ONLINE
 scrub: scrub in progress for 0h38m, 15.15% done, 3h35m to go
config:

        NAME STATE READ WRITE CKSUM
        tank ONLINE 0 0 0
          raidz1 ONLINE 0 0 0
            sdb ONLINE 0 0 0
            sdc ONLINE 0 0 0
            sdd ONLINE 0 0 0
            sde ONLINE 0 0 0
            sdf ONLINE 0 0 0

errors: No known data errors
root@utgard-2:/# zfs list
NAME USED AVAIL REFER MOUNTPOINT
tank 3.72T 3.43T 2.69T /mnt/tank/
tank/GPS-DATEN 3.88G 3.43T 3.88G /mnt/tank//GPS-DATEN
tank/apps 26.4G 3.43T 26.4G /mnt/tank//apps
tank/games 152G 3.43T 152G /mnt/tank//games
tank/home/ftp 29.6K 3.43T 29.6K /mnt/tank//home/ftp
tank/home/lan 33.6K 3.43T 33.6K /mnt/tank//home/lan
tank/home/share 694K 3.43T 694K /mnt/tank//home/share
root@utgard-2:/# zfs list -r tank/backup
NAME USED AVAIL REFER MOUNTPOINT
tank/backup 766G 33.8G 52.4G /mnt/tank//backup
tank/backup/apple 714G 33.8G 714G /mnt/tank//backup/apple
root@utgard-2:/# zfs mount tank/backup
zfs: lib/libavl/avl.c:637: avl_add: Assertion `0' failed.
Aborted
root@utgard-2:/#