Rear stand light on a Cube Nuroad FE not working I bought online a Cube Nuroad Race FE 2021 that comes full with fenders and dynamo lights. The front light is the Supernova E3 Pure and, although not stated clearly on Cube's site, the rear light should be Cube Pro-D dynamo mudguard rear light that includes stand light function according to Cube's site.
Now when the bike moves, both lights shine brightly and all is well. When I stop however (even after a long ride), the front stand light works, but the rear stand light does not. I noticed however a very very dim light in it.
I have asked Cube for help, but I am really wondering what is going on and whether there is a way to troubleshoot this. I know that this is not a bad component issue, since other users report the same behavior.
Also a minor note: I checked Supernova's manual and the only interesting thing I found was that the rear light should NOT be grounded to the frame. I checked it and could not see any ground cabling on the rear.

There are two wires going back to the rear light.
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.