So - after well thinking about it, I decided not to accept these color failures and could exchange my SP3! The shop was very friendly and easily changed it - they also did not see it before.
I think I now know how it happened - often when I work on the sofa I have my feet on the sofa and the SP3 leans totaly open at my knees. As I often wear biker jeans, i gess the hard jeans touched that specific place and as the coating there is weak, it was removed slowely with every move it did on my knees...
This time I will defenetly not use it that way again