The C4D section is here:
Your problem is simple. You have a bone in the arm that is not set up properly and is pulling on the polygons of the character’s torso.
I haven’t rigged in C4D for a while, but basically you avoid this by editing the bone’s envelope and painting proper weights.
Also when you model a character, do it in a T-pose like in this image: (it makes rigging less error-prone)
Ask in the C4D forum section. They’ll be able to help you.