The launch of the Wii U must be placing a significant amount of strain on Miiverse, since some users are saying that they’re unable to connect to the service. They receive the error code “121500″ when they make the attempt. If you’re experiencing this, I can’t imagine it’s anything serious. Just hang tight and hope that Nintendo gets it under control.
If you’re having more general connection issues — meaning, you can’t connect via Wi-Fi at all — then read this guide.