Not a Kerria, in spite of almost alike blossoms / Jedenfalls kein Ranunkelstrauch, trotz sehr aenlicher Blueten
If the shrub was overwintered outdoors in GER, I don't think it can be anything other than Kerria Japonica ‘Pleniflora’. It's the only shrub with that type of flower than would be hardy in cold winter climes.
Whatever it's called, it certainly has pretty flowers =)
That's the nice thing about having limited possibilities with a plant that bears nearly unique flowers =) And with this one, you really do only have two possibilities.