Finally I managed to compile the mtd-tools with the cross compiler. I created an image, put it on the plug, formatted the flash with it but now I cannot attach it.
The output looks like this:
~ # ./ubiattach /dev/ubi_ctrl -m 2
ubiattach: error!: cannot attach mtd2
           error 22 (Invalid argument)

while dmesg says this:
/ # UBI: attaching mtd2 to ubi0
UBI: physical eraseblock size:   131072 bytes (128 KiB)
UBI: logical eraseblock size:    129024 bytes
UBI: smallest flash I/O unit:    2048
UBI: sub-page size:              512
UBI: VID header offset:          512 (aligned 512)
UBI: data offset:                2048
UBI error: vtbl_check: volume table check failed: record 0, error 9

Can anyone tell me what might be wrong?

Thanks,
Markus

Now I tried a Transcend SDHC Card 16GB and no messages as well. I would be happy about an error message at least but nothing happens. Might the SDIO port be broken?

This is the message I get during boot time:
so the kernel should have the correct modules, isn't it?

It does not do anything. No error, nothing. It is new (I know that has nothing to say) so it should actually work.

My 16GB Extrememory SDHC Class6 is not working with the gentoo boot image. Anything I can do?