Umpa,
that's it. See his Pic 3, there you can see the snapped fence.
Now, what we can also solve the same is to check that the large cog has no axial play. When the wheel is loose, take of the metal part inside it, and refit it on the stub axle. NOW, keep it held down with the left hand, and move up/ down the white cog. IF it moves, look on the axle after taking it of. There are shims with which you adjust the free play. Grease this area, and if shims are missing, get some to adjust play to almost zero.
Reason is that IF there's play the white large cog can and will wander on the stub, thus the belt runs at angles, and the belt will not sit nicely on the small cog due to the angle it has to live with, then. I could not find the original 15 mm inner diameter shims so I got a 16 mm one, and lucky me it did the trick. I now have one thin original shim on top, all is fine.
Carsten