I entered my password wrong three times before I figured out that it was because the Miiverse login screen uses a completely different keyboard than the normal system keyboard that's used everywhere else. The vertical rows of letters are not aligned like they normally are (look on a computer keyboard, A is not directly below Q... it is down and slightly to the right). This was causing me to type where a letter should be, rather than where it actually was. Also, the shift key behaves like a caps lock key, which might cause you problems if you have a password with capitals in it. On the normal keyboard, there are two separate keys for shift and caps lock, but not on this one.
Awful, bizarre UI design decision to use a different keyboard. Can't imagine why they did that.
Anyway, just pay very close attention to what's happening on screen when typing your password.
Now it accepts my password, but I'm getting error 011-3051 whenever I try to get to eShop. Miiverse works.