After reflecting some time I can tell you what happend:
1. I had finished the tillering-job @ was very pleased with it (but didnt't do pics). Wood is rowan.
2. Then - in a hurry - I reviewed the tiller and found a stiff spot on the lower limb and started scraping again...it's where it finally broke @ FD on the tillering tree...
To my first surprise i didn't broke on one of the problem-spots I occurred while tillering.
3.But: I shouldn't have done this extra tillering in a hurry because the "stiff spot" in fact was a slightly reflexed wave in the limb. This overstressed the wood...BÄMMM...
Now I will go and clamp it with superglue
Cheers