Recently I've been having this problem when using Nintendo wi-fi for trading and battling either with Pokemon Heart Gold or with White version. I received during Christmas a range extender so it could boost my wireless signal since I'm too far to get good signal from my router(the Range Extender model is a TP-LINK TL-WA830RE for info), the router's firewall is disabled and it's a Thomson TG784n.
I wonder if this is getting interference with my router connection, because I was able to trade and battle before having the range extender (the signal was weak, but where I was I managed to use it during the time). For some reason I can use Game Sync on Pokemon White without any problem, but I guess since there's only a direct connection instead it doesn't make any problem with it. Just for trading/battling
If it's possible to clear this situation I would be appreciate, I tried several threads on the support forum at the official Nintendo website but with not much help, especially about manual setup's and static IP's. In case again the error is 86420.