Hi Damiano,
The mirror is not returning to its fully seated position after the exposure, hence the streaks (from the film winding on) and overexposure. Check the mirror flap carefully (and gently) for loose foam stripping on the back, etc. A 6006 is not young at this point, so it may just be dried, gummy lubricants.
If the images are also out of focus, check the seating of the magazine itself, and that you don't have a Rollei SLX film insert in the magazine as they don't fit properly.