Can't find a source but the closest one is here
Yeah, German speakers tend to learn Norwegian really fast, and with virtually no accent. I've met people who could be mistaken for a native speaker in six months. The only people who have an easier time are the Dutch.
You'd think the Swedish and Danish would have an easier time, and they certainly understand it easier, but they always end up with a heavy accent. A lot of them simply keep speaking their own language since the Norwegians understand most of it.