Solution: I had to resize the first partition (41 prep boot) to EXACTLY 4Megabytes. I had 32 Mb because I always used a boot partition which accomodated several kernel images and I thought is should be at least 4 Mb. Now, what a surprise... :-)
The kernel images are not copied to the PReP Boot partition, only yaboot is. Yaboot can then boot the kernels from whatever partition they are on.
Cheers, Jens.