If your Luxos U is out of warranty and if you want to try to bring it back to life, I found this link some time back with a google search.
https://groups.google.com/forum/#!to...on/qob5ex7lrWg
The 6/15/2016 post had several photos and a good discussion where the water was getting in. In that posting they talk about capacitors, but I think they are batteries and not capacitors.
I agree with pdlamb above on fenders. Many of the dyno powered lights have drain holes in the bottom that water can get into from the bottom, either from tire spray or from excessive use of water spray when cleaning.
I have not had any problems with my Luxos U but it has not been exposed to massive rain, only light drizzle.
For charging, I think the Sinewave Revolution (charger only, no headlamp) is one of the best out there for waterproofing. But that charger lacks the pass through cache battery that you are accustomed to having built into the Luxos U. I bought the SInewave for my bike tour in Iceland, never had any water problems with it even though I had a lot of rain. I did not have my Luxos U in Iceland, I specifically wanted really waterproof stuff on that trip.