Rear stand light on a Cube Nuroad FE not working
It would seem likely there's a problem with the rear light's capacitor. I'm a generator light user and nerd but not a real electronics person, so maybe someone else can answer better, but my understanding is that what you're seeing (everything works but the standlight is dim) indicates a capacitor that's barely functional. You could open the light up and try following standard methods to test and then replace it. Dropping in another fender mounted tail light is also an option, but they each have their own needs for where to drill the holes to get a good angle, so you might give up some of the integrated look of the stock light on that bike.