Can't play online with friend on the Nintendo Switch?

I wanted to play Overcooked 2 online with a friend. However, we always get a NEN error code (2618 0513, already googled, just makes no sense). Individually it goes online, but not if we want to play together. We do not know why that could be. Can this be related to being in a family membership? It would not make sense, but we can't explain it differently. In addition, we have each made the other console on primary, so that we share games (eg Overcooked 2) But it can't lie to it? We also tried other games like Splatoon 2 (everyone bought that themselves) and it did not work either. Internet connection is stable by both of us. Is it really because of the family membership or at the primary console?

Did you buy the games 1x or 2x?

1x. So he bought hats and I can play it too because of the primary console

Yes, just think about it. It is clear that you can't play it at the same time if you only bought it once, right?

Plain text: everyone must own it.

That does not make any sense. Then you could not even start the game. We can play it individually offline, only online does not work. The game sharing would make no sense otherwise