I really don't think it is any type of solidago. Perhaps horseweed (Conzya sp., or Erigeron sp. depending on which plant taxonomist you talk to), based on the branching of the inflorescence, the vase-like "flowers" (the involucre, more specifically), and the seeming overall ruderal aspect of the thing as Stephie suggested. Or maybe a fleabane (Erigeron in the narrower sense of that genus)? It's not any of the horseweeds I'm familar with from eastern North America, and I couldn't find a flora or checklist of Germany or northern Europe, but I would guess there are several horseweeds both native and exotic to where you live.
Edit: actually, maybe some sort of fireweed (Erechtites)? The leaves in the first picture make me think that more than Conzya/Erigeron, although it's not the fireweed I'm familiar with (hieraciifolius)