mounting and unmounting of btrfs filesystem on qemu qcow2 image mounted as NBD

Hi together,
I've created a btr(fs) filesystem on a partition on a qcow2 image with GUID partition table created with qemu (1.7.0) tool:
qemu-img create -f qcow2 image.qcow2 2T
I'm connecting this image to a NBD with qemu-nbd and mounting the NBD. I'm experiencing errors which I don't with partitions on the real HDD, e.g. <output /var/log/syslog>

Mar 24 20:38:48 myhostname kernel: [92324.728828]  nbd14: p1
Mar 24 20:38:48 myhostname kernel: [92324.759664] device fsid f
a920d4d-524b-4d17-86e9-bcb82b51cd1f devid 1 transid 2658 /dev/nbd14p1
Mar 24 20:38:48 myhostname kernel: [92324.759968] btrfs: disk s
pace caching is enabled
Mar 24 20:38:48 myhostname kernel: [92324.818021] btrfs bad tre
e block start 902103040 901578752
Mar 24 20:38:48 myhostname kernel: [92324.851668] btrfs: open_c

tree failed
</output>
Some images are broken. The image file is located on a NFS mount from another physical machine for which I haven't found any info on malfunction of NFS (I guess usual network errors are treated by NFS and qemu-nbd).

I'm addressing to the list not in order to get detailed debugging instructions, but rather a statement on whether it is possible to do what I want to do (mount a qcow2 image on a NFS mount) or why not. I'm not looking for alternatives :)

Further infos:
when I created the btrfs partition and partition table I did it on the NBD /dev/nbdX.

Thanks in advance for your help and pointers!

Best regards,
Karl Richter