Werra, it's the tank. I use 2 Jobo 2000 tanks and I've had that happen with E6. It's very definitely not a light leak, John R is right.
There's two problems with the 2000 tanks, first the reels ride up the spirals and that may definitely have happened here, there should be a locking ring, see attached photo, that prevents this happening. If you were using the lock ring then insufficient solutions, but 1 litre per spiral has always been fine with my tanks.
You can see by the edge marks, they coincide with the little guide bits that keep the film in place in the spiral, see second attachment, that it's definitely a process problem At some point in processing the spiral has been partial out of the solutions, exactly what caused the yellowish colour it's hard to say, maybe under development, contamination because the wash steps weren't effective on that top edge.
The answer is to use the lock ring, if you don't have one use a thick rubber band, make sure you use enough of each of the solutions, and use more than a litre at each wash step.
The second problem with the Jobo 2000 spirals is sometimes the sheets touch so be careful when loading to make sure that isn't happening.
I think you're saying you roll the tank on it's side for agitation, but what's causing this problem is while the tank is upright, make sure that you use inversions at the wash steps.
Ian