It seems, that not the boot image is the problem but the cpio initramfs.
I can use the command
abootimg -u boot.img -r initrd.img
to replace the ramdisk in a working boot image. This works only if a replace the current initramfs with the original initramfs. If I unpack the initramfs too and repack the exact same sources, it won't boot. The sources are binary the same, but the packed initramfs is totally different.
I use the following command to repack my initramfs:
find . | cpio -o -H newc | gzip > ../initrd.img