You may want to check with Peter White Cycles:

http://www.peterwhitecycles.com/

Peter is a distributor for SuperNova, he may warranty the light for you even if you didn't purchase from him. Peter White is in New Hampshire, would save the shipping to Germany.

The SuperNova headlights have some complicated circuitry inside, too complicated if you ask me. I read in the FAQ on the SuperNova site, said that they will upgrade the "software" in the light for free if needed. Apparently, there is some kind of programable IC chip inside. Cold be an issue with the circuit that switches between Dyno Power and standlight.