I could not solve the problem, but I found a way to shift around.

When creating a YAFFS2 image with mkyaffs2image, the files within the 
directory to create an image from does not appear in the root directory 
of the created image, they _exist_, but they are not shown. In deed they 
exist as directories (try a cd into them). But i could not delete or 
even list them (segmentation fault).

Is this a bug in mkyaffs2image?

A workaround is to

-> create an empty yaffs2 image with mkyaffs2image:

mkdir dummy
mkyaffs2image dummy img.yaffs

-> write into flash:

flash_eraseall /dev/mtd0
nandwrite -a -o /dev/mtd0 img.yaffs

-> mount /dev/mtd0

-> and write all files via cp into the filesystem

that works for me, but it is not the best solution

Christian