Hmm. Try repatching your ROM (Get the latest version of WFCpatcher!), and disable any cheat codes you have at the moment. (or are you using the AR code method? In that case, verify if your code is still correct)
And please be sure your connection information is still correct. (for instance, press the test connection button on your configuration. If this passes and says the connection is successful, you should be good to go!)
If all else fails, please tell us your decive model (eg.3DS, DS Lite) and your router model. @Snoof
For some reason I can use the matchmaking servers, get online & get into a race, but before the countdown starts I get the disconnect error. What am I doing wrong?
I've got a similar issue with another player a while ago. Was this you? My in game name was Reboot and my emblem is a pixelated Braixen favicon (yellowish fox thing).
What would happen is that the opponent disconnects before countdown and cause me to show that the player has disconnected.
Did you get an error code when you disconnected?