While on the subject, here is one of the other buffers (in this case, the front most). It was a cheap URO brand buffer. The metal part separated from the rubber. I believe it was this that put extra stress on the rear portion of the bracket leading to the fracture. Keeping your rubber buffers in good shape should prolong the life of the bracket.
This is a place to probably consider using on Genuine Mercedes buffers, which should be higher quality rubber pieces.