So we´ve tested a bit about Error 91010. The error appears at any time while playing or being online.This error shows up with the following message:
"Disconnected from network during transmission. Error Code: 91010"
This appears to only one person, the one who joined the room, and he/she gets disconnected from wifi. However, it´s possible to reconnect again with no problem.
The host of the room recieves a message wich says something like "You´ve lost connection with the other players. Returning back to menu" (Will edit as we test) and he/she goes back to the online Brawl with Friends menu.
This happens between two NSCT ISOs as well as between a NSCT and a PAL ISOs. We´ve tested with and without DNS activated, and we get the error in both cases.
If you actually search in the Nintendo Support Page, it seems that this error is caused because of the Wifi shutdown.