Mounting nbd0 during bootup

[g <sendmailtogoran@gmail.com>]

I'm trying to mount /dev/nbd0 during boot. All went fine with this config

[generic]
    user = nbd
    group = nbd
[blade]
    exportname = /srv/nbd/blade.img
    trim = true

But I need to use the image with hundreds of clients as COW-mount. So I used
this config

[generic]
    user = nbd
    group = nbd
[blade]
    exportname = /srv/nbd/blade.img
    copyonwrite = true
    sparse_cow = true
    trim = true

But with this I get errors

EXT4-fs (nbd0): ext4_check_descriptors: Block bitmap for group 0 not in group
(block 1442887680)!

How can this be?

Regards
Goran