Many LAN Adapter (both Nintendo and Datel) users are reporting that, after the update, they are unable to connect to the Wii Shop Channel and News Channel, with error number 51420-9. All other online features stay operational.
I'm included in that number.
Does this fit your problem, Sméagol?
It seems like Nintendo hasn't really tested this update properly before releasing it...
The workaround is to use a wireless connection, but that doesn't invalidate that the update has made many LAN Adapters partially useless...