Its about Nintendo clouds service.
I even got an mail some month ago telling me its about this.
They have something like a cloud service and akamai, where the traffic is transfered with less lag to the US Servers, as they dont have an own connection to big internethubs in Frankfurt,London,Paris and Amsterdam.
Anyway, they get overloaded by traffic in Europe and the east, thats it.
besides, if you just get Black Screen with "connection error", it means another user dropped out of the game in Community Mode (2-Players only) or that the server kicked you, for whatever reason (driving more than 20 seconds wrong way etc).
If you don't have problems with other Games and other Consoles, than this should eb your answer. Besides, it should also be indicated that its not you or your router, as you can connect to the Nintendo Network and you just get this Problems while racing or entering a Room/Game.
The other thing to say is, that some providers seems to have less problems than others, Vodafone in Germany for example has more problems, than users of T-Home or Telefonica.
The best thing you can do is waiting or playing at different times, Nintendo seems to raise the capacity all 1-2 Month.