Before I went to an Epson V500 I was using an Epson perfection 4180 and after some time the lights in the lid that are used to illuminate the negative dulled a bit and would leave scan lines EXACTLY like these.
If your not scanning the negs then it will not be this but if you are just have a look at the illumination.
The Epson V500 does not have this problem as it used a different set up and the illumination lights move with the scanning head.
The scientist in me would lean toward testing to find the source of the problem. I'd do two things: 1) Have the film from your Rolleiflex developed at other places and see if the same problem occurs. If it does, then the problem is likely with your camera. 2) Have film from other cameras developed at your normal place. If the same problem occurs, the problem is likely with the lab.
The same issue happened with film out of my Yashica D. It was Kodak 400 Tmax and was given back to me with the same rolls from my Rolleiflex. That is what leads me to believe that it was a problem with the lab.
I used to work in a busy photolab and when films showed signs of fogging/light leaks it was normally due to the film not being properly loaded in the processor. Even by not slamming the door to the processor hard enough caused these sporadic light leaks to occur and as they were so slight it was easy for us (the lab) not to have to accept responsibility...I mostly used to load 35mm and 120 films into the processor and the 120s were the worse as they had to go into a separate loader container and were particularly fiddly. I am (definitely) not an expert but I would guess that the problem occurred with the lab.