This may be a little difficult to pinpoint the exact cause. Looks to be a light leak in the camera. Odd that it doesn't show on the other 11 frames. How straight/square is the camera back? Rollei backs can be bent in use and you won't notice it. Why the 12th frame only, could be that since it is the tightest wound part on the roll it has enough spring to cause a light leak, older/outdated film can get quite springy. Have you opened the back and dry-fired the shutter to look for leaks. If you still can't find the source then a trip to the repairman is in order.